mixer settings good? main volume up? 'wave output' volume up?
How is the sound board plugged in to the stereo? Check and make sure you didn't plug the cable into line-out-#1, and not the line-in, microphone-in, or line-out-#2.
Just to clarify, it's possible to get weak sound coming out the microphone jack, and maybe even the line-in sometimes... I accidentally plugged something in this way once and got real weak sound. I was like "what the heck" until I saw the line input I'd plugged the sound board into start smoking. (was a cheap car stereo with a line-in, and a laptop microphone jack was patched to it. Stereo didn't like that much.)