Cannot have sound in multiple apps?
 

Sometimes I'm playing Runescape (a game that runs as a java applet in a browser) I like to have MP3's playing in the background, but still have ingame sound on. This works fine on the Mac and Windows, but Debian doesn't want to play nice.
Whichever app I'm running that plays sound first takes over the sound system and blocks the latter from playing any sound at all. For instance, playing Runescape in Iceweasel causes whichever program that I load afterwards to play an MP3 to not play. Likewise if I'm already playing an MP3, Runescape won't generate any sound.

I've tried OSS, TOSS, ALSA and ESD - none of them seem to allow me to have to programs producing sound simultaneously. Is this a limitation of Debian/Linux? I'm fairly new to Linux, but I would have though that this was a rather major limitation. Well, ok not major but rather lacking...

If it shouldn't be behaving like this, what commands must I use to generate info for you folks to analyze?

You need a sound server running. There are several. aRts is the standard one most KDE distros install, but aRts is not perfect. There are others, so if your sound card is working, all you should need is a sound server. What it does is accept input from different sources, and schedules them through alsa.