Yes they are. Technically though it's not mixed. All arriving audio data is dumped into a single buffer. Mixing would now be a great way to get rid of the stream's choppyness.

now that so many users are playing simultaneously, I'm a bit annoyed that some leave the website open and stream silence.

Wondering how I could get a user a fair slot to perform now...

Anyways, the stream has become a great source of entropy now :D