Assuming Win XP...

Go to "advanced volume controls" and make sure that all is well there, too (there are more settings). Then there's speaker volume ("change the speaker settings"). I have a feeling there are other hidden volume controls as well, but I can't remember what or where they all are. I seem to recall one that let me set my speakers to sound as though they were in a "small room," "cave," "cavernous concert hall," stuff like that. Can't find that setting right now.

The "Sounds and audio devices properties button" has a tab called "hardware" that appears to be able to tell you what's working and what ain't.

And you might try the sound troubleshooting guide; it helped me once with a particularly gnarly problem.

I am sure there are many others here who will give better advice...

Good luck!

C