Probably (although not certainly) it is because some other program has "grabbed" the soundcard and you only have a single channel soundcard.
This is often the case if you are running a "sound server" which uses the OSS drivers, but your apps use the ALSA driver. Forget about those terms for now. Before I can give specific instructions on what to do, you need to provide a little more information:
- What distribution do you use (e.g. redhat, suse, mantriva, ubuntu, kunbuntu etc.)?
- What sort of machine is it - a laptop, a desktop, a server, non-intel?