[lch]

That depends on your system. I have no idea about Windows. It should either work magically, or not at all like you want.

I guess you are on a Mac. I have no personal experience with them, but AFAIK the system is structured somewhat like Linux. I can only speak about that: You have to look what sound daemon your system is using. Under Linux, Dom3 can use the aRts sound daemon (and it's the default) or the OSS sound daemon ("for better quality"). I think the OSS sound daemon is not capable of handling two sound sources at the same time, it locks the sound device for one source, at least on my system. The aRtsd however can handle two sound sources at the same time and has no problem to mix the sounds from the sources. So you can play something in the background and still hear the sound effects from the game.

Just make sure that all your programs are using the same sound daemon as output device. Check what audio options your Dom3 executable is offering with the "-h" switch, the audio options should be at the bottom.