Check in view-->options-->sounds--> requests. Do you have accept sound requests checked? Same place, under sound files, do you have the directory filled in where you keep your mp3s? While you're there, may as well check in options/sounds to see if you have enable sounds checked.
Obviously we have no way of knowing what the scripts you are using have done to the file names or how they effects mIRC. Talk to your friend and you can both look in your mp3 directory to find one that has the same name to test with. Remember that if you name a file tweedledee and your friend names it tweedle_dee then as far as your puter knows, they are two completely different files. As a side note, mIRC itself doesnt play wavs/mp3s, it sends the /sound request to windows which then sends it on, like to mediaplayer