Stereo or 5.1 for web playback?
Stream stereo. Hand over the 5.1 file.

Nothing in the formats stops a 5.1 track from riding inside a web video. MDN’s audio codec guide lists AAC at up to 48 channels plus 16 low-frequency channels, and Opus at up to 255. Those are the limits of the codecs. They say nothing about how a particular browser routes six channels to a laptop’s two speakers or a client’s headphones.

YouTube’s own upload recommendations point the same way. The channels it lists are Stereo, Stereo + 5.1, or Stereo + Eclipsa Audio. There is no option without stereo in it. It recommends 384 kbps for a stereo track and 512 kbps for 5.1, at 48 kHz. Those are settings for what you upload, not a description of what each viewer hears.

One such rule is documented. MDN describes how the Web Audio API folds 5.1 into stereo when an audio node’s input and output channel counts differ: the centre channel is added to each side along with that side’s surround, at about 0.7071 of its level, roughly minus 3 dB, and the low-frequency channel is dropped. Written out, left becomes L + 0.7071 × (C + SL), and right becomes R + 0.7071 × (C + SR).

Read that as one documented rule, not as a promise about any browser’s video playback. It governs Web Audio nodes, and our player does not use Web Audio. What it does show is how much a fold-down decides: the balance of dialogue against the surrounds, and whether anything placed only in the low-frequency channel survives at all. Under that rule it does not.

A stereo mix removes the question. The client hears the balance you signed off, on any two speakers, and a note about the dialogue level is a note about your mix rather than about their setup.

Browser playback uses your uploaded export without another platform re-encode. Codec, device and connection requirements still apply. For audio that means no re-encode, no resample and no downmix: a 5.1 track is stored and streamed as six channels, and a stereo track as two. The player is a plain video element. Its only audio control is mute; it does no channel handling of its own.

When you upload, the file check reads each audio track’s codec, channel count and sample rate. The uploader shows you the codec label only. The channel count is not displayed, not checked against anything and not changed. If your own browser does not report support for the audio format, the uploader says so and offers to upload the original anyway.

So we will not tell you a 5.1 upload plays in surround here, and we will not tell you it folds down well. We have not tested 5.1 playback per browser or device. If the whole point of a review is hearing the surround field, a browser link is the wrong room: play the file on the speakers it was mixed for.

The full list of what else belongs in a handoff is in the video deliverables checklist. If your stereo mix already sounds thin on other platforms, the cause is usually their re-encode, covered in why audio sounds worse after upload.