Did you try the CD player in another system?
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I think you might have a bad cd player. You know that the tape input works because you said the Onkyo cassette deck sounds okay. Try puting the cd player into the tape input using the same cable that hooks up the tape deck, (that will eliminate a bad cable). If it still sounds bad, your cd player is at fault.
The tuner, aux and tape inputs are all the same, so it really doesn't matter which one you use. The phono input is for turntables ONLY, do not use it for anything else.
I doubt it will fix your problem because you say that your other sources sound OK, but its worth a try. If your voices are not "locking in" as you say, you may have them wired out of phase. Go to 1 of your speakers and reverse the speaker cables at the binding post. Put the + where the - currently is and put the - where the + currently is. Do this on 1 speaker only and they play some music. If this fixes the problem, you will hear it immediately.
Also, you may want to have a look at your NAD. Its very common to find a solder connections go bad; usually around the RCA connectors.