Currently we don't support 5.1 audio at all (see bug 521615). However, even if we tried to open a 5.1 output stream and get the channel mappings correct, some platforms (such as Android) will never be able to provide more than 2 output channels. We should add support for downmixing such streams to what the OS actually provides.
Many Android devices pass 5.1 multichannel audio through HDMI.
Currently Firefox is now the only major browser not supporting multichannel AAC audio.
Maybe we can get past this problem soon.
We provide high performance high quality streaming audio encoders to many of the largest content providers. We have test HLS streams available upon request. The surround streams unfortunately, play in 2 channels using Firefox. We need multichannel AAC decoding! This would be easy to do now that Firefox is MSE compatible.
Our test multichannel AAC HLS streams currently play using Microsoft Edge, IE for Win 8/10, Safari Mac OS X and iOS, and Google Chrome. It sure would be good to add Firefox to this list.
Please let me know if we can be of assistance to make this happen.
Here are test resources:
I believe we do support downmixing (for some time now). We don't support 5.1 through the graph to cubeb output, though, so revectoring this bug to the cubeb portion of that.
We'd love support for 5.1. Greg, if you have some cycles to work on patches for cubeb for this, that would help move this feature along, as right no all our audio people are pretty tied up with other work (full-duplex, Worklets for WebAudio, etc). We can help guide you through the process of submitting patches. We all hang out on IRC at irc.mozilla.org in #media.