X's developer documentation states two hard requirements for the audio track in a video: it must be AAC with the Low Complexity profile, and it must be mono or stereo. A file that misses either one is outside what X says it supports, and that is the first thing worth ruling out when a posted clip plays silently.
One caveat before you start changing files: X states these as requirements but does not document what a violation looks like from the viewer's side. There is no official error text for a rejected audio track, and no official statement that non-conforming audio comes back silent. So treat what follows as a checklist of things X has told you to satisfy, not as a symptom-to-cause map.
What X actually requires from an audio track
The constraints live in the Advanced section of X's media documentation. Verbatim:
"Audio must be
AACwith Low Complexity profile. (High-EfficiencyAACis not supported)"
"Audio must be
monoorstereo, not 5.1 or greater"
Both sentences do something unusual for platform docs: they name the thing that is excluded. High-Efficiency AAC is called out by name as unsupported, and 5.1 is called out by name as too many channels. If your encoder offers a choice of AAC profile, that parenthetical is telling you which one to avoid.
The same answer shows up on two other X properties. The Media Studio Library encoding spec lists "Audio specification: stereo/mono AAC (LC)". X's advertiser creative specs list "Audio codec: AAC LC (low complexity)". Given how often X's pages disagree with each other on duration and file size, three properties converging on the same audio answer is worth noting.
For the level rather than the format, the developer docs' recommended settings give:
"Minimum Audio Bitrate:
128 kbps"
"Audio Codec:
AAC LC"
Note that the same page also carries an upload recommendation table listing audio bitrates of 128K, 64K and 32K depending on resolution, introduced with the line "In the table below each row represents an upload recommendation, but is not a requirement." Two sets of audio numbers on one page, one of them explicitly not binding.
The two exports that produce non-conforming audio
Neither of these has anything to do with file size, which is the axis most people check first. X's size and duration caps are a separate subject entirely — see X's video file size limit if that is what you are actually chasing.
- A High-Efficiency AAC track. X's wording is unconditional: High-Efficiency AAC "is not supported". Some encoders offer HE-AAC as a low-bitrate option, and picking it is easy to do without noticing.
- A 5.1 or greater mix. Multichannel audio coming out of a camera, a field recorder, or a timeline that was mixed for surround puts you past "mono or stereo".
The fastest fix: re-encode to a plain H.264 MP4 with stereo AAC
There is no way to inspect X's side of the pipeline, so the efficient move is to eliminate both constraints at once rather than diagnose which one you tripped. Re-encoding the file to H.264 video in an MP4 container with a stereo AAC track satisfies the format requirement, the channel-count requirement, and X's recommended AAC LC codec in a single pass.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
This runs inside your browser tab — the file is read from disk, re-encoded locally, and handed back, so there is no upload to sit through before you get to the repost. It outputs H.264 MP4 with stereo AAC, which is the combination X's own recommended settings describe.
Other things to check, in order
Is the audio missing, or is playback just silent?
Before re-encoding anything, confirm the track is actually gone. X's documentation describes playback as having two states, and ties caption behaviour to them:
"By default, subtitles appear when the video's sound is not playing, and will be hidden when sound is playing."
So if captions are overlaid on your clip, X considers it to be in the sound-not-playing state — which is a playback state, not evidence that the file lost its audio. Check the same post with sound deliberately turned on, and on a second device if you can.
To check the source file itself, pull the audio out and listen to it on its own: extract audio from video gives you a standalone track in a few seconds. If that is silent, the problem is upstream of X and lives in your editor's export or your source recording.
MOV sources
X makes one container-specific recommendation, and it is about processing rather than audio:
"This happens most often with .mov files. If possible, re-export files as .mp4, which should allow for overall faster upload and processing time."
That sentence is about uploads stalling, not about sound, so do not read it as an audio fix. But if you are re-exporting anyway, MP4 is what X asks for — MOV to MP4 does the container change without touching anything else, and MOV uploads stuck in processing covers that failure mode properly.
Encoding settings are already on X's own failure list
X lists "encoding settings" among the causes of a failed upload, alongside file size, resolution and connection issues. The same FAQ contains a warning worth remembering here: "Error: 'Video too long' — This error usually appears when uploading media that isn't in a supported format." X's error text does not always describe the actual problem. If you saw any error at all during upload, X upload failed works through the full list.
One scope note: that FAQ opens by stating the instructions "are only valid for Media Studio, and should not be followed for usage of the X developer API, Pro Media API, desktop upload, or mobile upload."
Live streams are a different spec sheet
If the silent video is a livestream rather than an upload, the numbers come from elsewhere. X's Live Studio encoder table recommends "AAC (mono or stereo)" audio at 128 kbps. X's advertiser pages list the same row as "Audio codec: AA-LC" with "Audio bitrate: 128bps (maximum)" — quoted exactly as X writes them.
What X does not document
Three things you will not find an answer to, so do not accept one from a third party:
- What happens to a non-conforming audio track. X says what is required; it does not say what it does with a file that misses the requirement.
- A complete list of supported formats for web or mobile upload. X's help page only says you will be prompted if the video is not in a supported format.
- Whether X alters audio during processing. X does say "we may modify or adapt your original video for distribution", and names "modifying the resolution and bitrate of the original video while streaming" as an example — resolution and bitrate, with no mention of the audio track.
For anything in that list, check the official documentation rather than a forum answer.
How to tell it worked
Check the re-encoded file on disk first: play it locally with sound on and confirm you hear the track, since that is the file X will read. Then post it and watch it back with sound explicitly enabled rather than relying on whatever state the timeline puts the player in. If the new export has sound on X and the old one did not, the difference was the audio codec, profile, or channel count — not the size of the file.
FAQ
Why does my X video have no sound after posting?
X requires audio to be AAC with the Low Complexity profile, in mono or stereo. A High-Efficiency AAC track or a 5.1 mix is outside what X says it supports. X does not document what a non-conforming track produces on playback, so check your file against those two requirements rather than trying to match a symptom.
Does X support 5.1 surround sound in videos?
No. X's wording is "Audio must be mono or stereo, not 5.1 or greater." Downmix to stereo before uploading.
Is HE-AAC audio supported on X?
X states it is not: "Audio must be AAC with Low Complexity profile. (High-Efficiency AAC is not supported)". Use AAC LC instead.
What audio bitrate should I use for X?
X's recommended settings list a "Minimum Audio Bitrate" of 128 kbps. The upload recommendation table on the same page shows 128K, 64K and 32K rows, but X introduces that table by saying it "is not a requirement."
Why do captions show up on a video that is playing silently? That is documented behaviour, not a bug: "By default, subtitles appear when the video's sound is not playing, and will be hidden when sound is playing."
Does converting MOV to MP4 fix missing audio? Not by itself. X ties the MOV recommendation to upload and processing speed, not to sound. It helps only because re-exporting is also your chance to write a conforming stereo AAC LC track.
If you only do one thing, re-export with stereo AAC audio in an MP4 container and post again. More upload and playback fixes are collected in the fix guides, and every browser-side converter and compressor is listed under all Clapr tools.
Quotes above are from X's developer media documentation, the Media Studio Library FAQ, X's Live Studio help page and X's advertiser creative specs, checked 16 September 2026. X's pages have carried conflicting figures before — verify against the official documentation before relying on any number.
