It doesn't look like you can change those sounds. The information for custom UI sounds is in the UIBuilder documentation or here:
http://www.eq2interface.com/forums/s...ead.php?t=1703
http://www.eq2interface.com/forums/s...ead.php?t=1787
I would guess that the problem is the piercing high frequencies. Have you tried turning down the highs in the sound control panel EQ?