This is the obvious question, and I'm sorry to ask it, but this is the first step:
Did you turn the volume up? Just one volume up, or all channels? Sometimes the "master" channel in the GUI doesn't control the actual master volume. Also, there can be multiple volumes controlling the same channel (master+pcm, etc). Make sure they are all turned up. Again, I don't mean to call you an idiot or anything; you would be surprised how many people forget to check this.