Firstly you need to identify whether this is a hardware or software problem:
Something may have gone wrong whilst you were installing XP, try formatting the hard drive and installing XP again.
If theres still no sound trypartitioning the hard drive and reinstalling 98 on the new partition, and see if you have sound on there.
If none of the above work try using a different PCI slot. It may also be a problem on the motherboard, or just an incompatability problem.