I can only guess that the way mIRC stores sound files internally is not working correctly under Windows 7 64bit. Hopefully someone else who has Windows 7 64bit will be able to test this to see if they can reproduce it to confirm the issue. The only option in that case would be for you to turn off the internal beep option to make mIRC use your default system sound. If it does turn out to be an mIRC issue, I should have it fixed in the next release.