Ok, took a look, and a correction (and additional info for you). I will get the wiki updated with this, and verify all other info for it (commands, etc.).
It actually uses .wav files, but if you want, I could make it also allow .mp3 if that makes it any easier for you, wouldn't take much work on my part (in fact, I can just migrate the code I use in MQ2GMCheck to do it).
The sounds need to be in your MQDir\Sounds directory, e.g. if you have MQ2 in C:\MQ2, then the sounds go in C:\MQ2\Sounds. Our zip/installer has such a directory.
In that directory, it will use these files for the sound options, if sound is enabled in the plugin:
Reminder: pcremind.wav
PC Enter Zone: pcenter.wav
PC Leave Zone: pcleave.wav
htw