I would not have thought that this is a common enough problem to merit reporting here, but I have now seen several emails on this subject: if your Mac's sound works fine except that you do not get any sound from audio CDs, open up your Mac and check if the special audio cable going from the CD drive to the motherboard is connected. If it is loose, simply plugging this cable back in its socket should yield success.
Note: This is only one possible cause of this symptom. Most other causes deal withvarious incorrect software settings (such as for Sound Input). However, if all else fails to work, consider this cable problem as a cause.