This is something that worked perfectly fine in Firefox up until a day or two ago and then it stopped working. That is upmixing the stereo feed to 7.1 surround stereo (obviously not true 7.1 surround but sound came out all 7 speakers and the sub). Now it has stopped working on Soundcloud in Firefox, whereas it continues to work without any problems on Mixcloud, YouTube, and any other media sites I visit with Firefox. It has never worked properly in Chrome with any media site, so Firefox has been my go to browser for listening to streamed music.
So clearly something has been changed with Soundcloud for this to have only stopped working on this one site in Firefox. The sound is now exactly like it is coming out of Chrome, 2 speakers only with very thin bass.