You have to do two things to make sound shared:
- Modify your asound file (.asoundrc in my case) to tell ALSA to share
- Modify your software settings so they all use ALSA.
My XMMS, for instance, has ALSA, OSS, Disc Writer and eSound as possible outputs. Mplayer has mpegpes, OSS, Alsa, aRts, eSound, SDL, null, and PCM as possible options.
And Gaim is even worse - it needs to be set to "Sound method: Command" and "Sound Command: aplay %s"
So, make sure all your apps are using ALSA, and make sure your asound file is correct. FWIW, mine is: