Thanks for the bug report, unfortunately I am unable to reproduce this issue here under Windows 7 64bit with the latest release version of mIRC v7.25.

The following script looks in your sounds folder for wav files and plays up to 10 files almost simultaneously. If you add the following script to your aliases section and then run it with the command /test, are you able to reproduce the crash?

Code:
test {
  var %dir = $mircdir $+ sounds
  var %n = $findfile(%dir, *.wav, 0)
  if (%n > 10) %n = 10
  var %m = 1
  var %d = 100
  while (%m <= %n) {
    var %file = $findfile(%dir, *.wav, %m)
    timer -m 1 %d /splay %file
    inc %d 100
    inc %m
  }
}

The most common cause for a crash when playing a sound is your Windows sound drivers. Some 64bit sound drivers seem to implement support for the Windows Media Control Interface (MCI) library in a way that is not reliable. Which sound card/drivers are you using? You should make sure that you update your Windows sound drivers to the latest version for your computer. You may want to try uninstalling them first through the Windows Control Panel Device Manager and then letting Windows reinstall them through Windows Update.