If you have the "internal beep" option enabled, mIRC will play a sound that is stored in mIRC itself, so there is no need for a separate sound file. I just tested mIRC v6.35 under Windows 7 32bit and it played the internal beep sound without any problem. If you disable the internal beep option and then test the beep sound, does it work? You should hear the default Windows sound in this case. I cannot think of a reason why it would not play, unless there is a specific issue with mIRC playing internal sounds and Windows 7 64bit, however I do not recall anyone else having reported that before.
