I solved the problem. The AC'97 codec doesnt have hardware mixing in linux. Fortunately there are various sound systems in linux like EsounD, ALSA, OSS, and Arts. Players like xmms and mplayer and Amarok can use several and so it still is possible to hear more than 1 sound at a time. Arts can even do software mixing and comes with KDE. There is also OpenAl, a sound system for games like Unreal tournament 2004, that uses software mixing. Some programmes, however, dont have support for these various sound systems and take over the only channel available. An example is the game Chromium. In that case you cant have anything else running that makes noise.