A while back, I got really annoyed from the fact that when I was running Gentoo and I decided to play a game (Wine/Cedega) like EVE Online, I either have the sound from EVE-Online, or from my desktop - but not both.
This is usually due to a crappy sound card (no hardware mixing) and a driver which fails to fix this. I knew ALSA had dmix for software mixing but I read somewhere that pulseaudio was the new hot shot in the mixing scene. I was wrong.
You see, after enabling pulseaudio in Gentoo, rebuilding a number of packages to support it (Xine-lib for example) and following the guides to make ALSA-only applications use pulseaudio as well (in fact by looping the sound back from ALSA to pulse and again to ALSA), it should just work (tm).
My first attempts were fine, I used aplay to select the pulse channel, played 6 songs at once and all was well. When it was time to make Phonon (KDE4) use the new pulseaudio sink, nothing happened. I even used the workaround as explained on the pulseaudio page - but nothing helped. I finally made the default audio-sink pulseaudio.
Next up: Wine. The first attempt to start Winamp went fine, except for the fact that I didn't hear no sound. Looking in the logs, I see wine attempting to open the ALSA/Pulseaudio library from '/lib32'. For the more perceptive of you: yes, it want 32-bit libraries on my 64-bit system. Normally this is handled by a emulation package but guess what? The Gentoo emulation packages don't feature these only slightly vital libraries.
After reading the bug page which explained the problem, I found a site of someone who provided the 32-bit binary drivers for ALSA/Pulseaudio. And indeed, after extraction into '/lib32', sound came to life from the mysterious world inside Wine.
Then I tried to start World of Warcraft. The stuttering sound of a screwed up sound mixer emerged from my speakers and no matter how I started the game, the sound was useless. Finally I gave EVE-Online a try and it had the same, horrible sound output...
So I reverted everything I did, read up on how dmix was supposed to work (which seemed like a lot less work after following the 'simple' guide on the pulseaudio site), set up a software mixer and made it default and there we go - audio output for all!
Now I don't know who decided that pulseaudio was 'lightweight' and 'fast' but I know for sure that its neither - the default dmix extension for ALSA works a million times better...