i'v played this game for the last time 2 years ago, and started to play it again this week. everything was all fine, and it didn't do anything strange since i now have vista on my computer.
but recently the game began to bug, the ghosts don't make sound anymore when they try to help you free them. i can't hear them, and other sounds seem to be disabled too. but the intro on each level does fine. it is really only the ghosts, so i can read te subtitles, but that isn't too great... this just happened mid game and doesn't go away anymore.
You might want to take a look in the Ghost Master data folder (C:\Program Files\Empire Interactive\GhostMaster\Data). See if you can find any .snd files (just do a search). These are mostly voice files. If you can't find any or all file sizes are zero, reinstall the game. Also check your in-game settings to see if the sound hasn't somehow been muted. Sorry about the late reply, by the way.