Does the MP4 HTML5 compatible video type allow for mono audio? I'm doing some rendering in Vegas Pro and notice that if I select MainConcept (which gives me an HTML5 compatible MP4) the audio tab only has options for the bit rate and the sample rate. Before rendering, I combined both left and right channels on the timeline, but once rendered, the video file has two audio channels.

This is normally not a problem, but in this case, the audio is only voice and I'm concerned about file size, so if I could render with a mono channel, I would be able to get a higher bit rate for audio quality, but keep the file size small.

What Vegas is doing is taking the single channel audio from the timeline, duplicating it, setting a left and right channel, then cuts the set bitrate in half for each channel. So if I want 96kbps mono, I actually end up with  stereo, 48kbps on each channel.

Is this an inherent issue with MainConcept file types? I notice that with others I can actually select mono in the audio tab, but not with MainConcept. I thought I read a short half sentence saying as much on some forum, but not sure.

If there is a way to have an HTML5 compatible MP4 with mono audio, please tell me how.
The Mainconcept or Sony codecs in Vegas are subpar for low-bitrate output such as needed for HTML5 use. Use ffmpeg to generate your HTML5 videos. Yes, mono audio is acceptable for HTML5 use, but once you use ffmpeg/x264 for generating those videos, the 6kB/s savings you get from switching to mono audio won't matter much if at all.

Get the 32-bit static build of ffmpeg from here. Render a high bitrate Mainconcept MP4 from Vegas, say, 12 Mbps. And then run the following ffmpeg command:

ffmpeg -i "vegas-output.mp4" -crf 23 -c:a aac -b:a 96k -strict -2 -movflags +faststart "html5-video.mp4"


If you still want mono audio, insert

-ac 1


after aac.

If you don't like the quality or size is too large, then play around with the CRF value. They go from 0 to 63 with lower being better. 18 to 28 is the usual range you want to stick with.