Thanks for your bug report. It looks like the Windows MCI API has a filename length limit of around 100 characters. When you specify a filename length longer than that, the MCI API fails. Due to the number of issues with MCI in previous versions of Windows, a sound playing backup was implemented whereby if the MCI failed, mIRC would revert to playing the sound using a simpler API, to ensure that a sound played in case it was important to alert the user, however the simpler API has no support for controlling sounds. That is what is happening in this case.