Music and program sounds are a little distorted and crackly unless I turn the PCM volume down to about 70%. But when I listen to music in amaroK, the PCM volume goes back to 100% when the song changes and I have to turn it back down. I think I might need drivers for my onboard sound. I have an Acorp 4S648FX motherboard.

That is a possibility. Your best bet would be to go to your motherboard manufacturers website and find out what sound chipset you have. After that you can go to Advanced Linux Sound Architecture - ALSA and check the hardware compatibility list to see if your sound card is supported under alsa. If it is there are directions for correct install on that website.

There is also a chance that your sound driver IS installed correctly. I run an Audigy 2 on my PC and I have the PCM volume around the same level (70%) that you do so I do not receive distortion. As far as amaroK during the PCM volume back up there should be a setting so it adjusts MASTER volume instead of PCM. That should solve that problem. I have no experience with amaroK but with other players I've seen settings like this. Hopefully this helps and good luck.