I am trying to help a disabled client who has installed the Underwater theme on his Win98SE PC. Sometimes as he opens each Menu (from Start) it pauses very briefly and then the Open Menu sound has a click in it. He has 512Mb of memory, loads of disk space and a new sound card we bought to try and resolve the problem. We have set his volume controls high and low to no avail.
Sometimes when using a program which speaks the instructions (from WAV files) that also has the crack/click in the sound.
Have a look in the Underwater theme folder, probably in C:\Program Files \ Plus! \ Themes, then look for a wave file probably called MenuP, menu popup or something. Rename it, move it or delete it and the sound should go away.