This is absolutely unacceptable, and the main reason I still use a SoundBlaster Live instead of my onboard sound. The SBLive can do hardware mixing and the Linux driver makes this transparent, so even OSS programs can run simultaneously.

ALSA is supposed to be able to do this transparently (in software) with dmix, but in my experience that only works with ALSA-aware programs, not those using the OSS API. I don't care that on Linux OSS is deprecated: it should just work.

In short, Linux sound is still obnoxious unless you are lucky enough to have hardware mixing work, or you only use programs which properly deal with ALSA.