I don't think audio applies here at all.

Each channel can be mixed in parallel, 44,100 samples per second is not much per channel and mixing isn't difficult.

Also most can be cached and don't need to be mixed or filtered in real time because they haven't been changed from the last play.

Sorry, I should have been more specific. I meant DAWs and computer music environments, not simple audio players. Modern DAWs are heavily multi-threaded.