No sound from internal CD

I have a PC with motherboard with sound built in, which doesn’t have a connector/socket for me to wire the sound lead from my internal CD-ROM.
I also have an external CD drive (actually an EMPREX USB IDE drive container which I’ve put a DVD-RW drive into).
I can play CDs in the external drive (connected via USB) but although audio CDs are recognised in the internal drive and appear to play, I get no sound.
Is there a way round the lack of a connector for the internal CD sound lead? (And how come the external drive does allow me to hear CDs over USB?)