Weird Sound Problem

recently we started to get computers that had problems playing sounds after upgrading from Windows 98 to Windows Me. We have since overcome that problem and are now at a new dilema. Recently we came into contact with a RealTek AC97 integrated motherboard audio system. The computer didn’t have windows media player installed on it and we installed WMP 9, which was our first mistake. We rolled back and installed 7.1. We cannot get WMP or any other program for that matter to play .wav files. Yet system wav files play and .mp3 files don’t play either.

Any thoughts?