Which signal is analog? That is usually a grounding issue. But with analog to digital converters, you may have a different issue. Are you using a MAC for your music source?
Let me get this straight. You are using the PC speaker output to connect to your mixer?
If this is the case, then your problem is right there. You do not want to use a variable high level output to connect to the mixer. You need low level outputs. You need a soundcard that has a stereo RCA output. Then you connect that to the mixer.