I can't seem to reproduce this issue here. I installed a clean copy of mIRC, opened the Options dialog, Sounds section, and modified the selections in "On Event, play sound", and pressed the OK Button. I then re-opened the Options dialog, Sounds section, and scrolled through the items in the same listbox. It looked fine here.
Which version of Windows are you using? Have you tested this with a clean install of mIRC?
