JTT, I have noted you only say that the sound has gone so I assume that the CD is operating normally otherwise. Is this drive an internal drive? Assuming you are using a PC (I know nothing about Macs), if it is an internal drive, are you sure that you did not dislodge the cable that runs from the CD to the sound card while fitting it?

Other questions:

Can you play other sounds, e.g. wav files and MIDIs?
Does CD player appear to be playing the CD?

Jon