It's possible for a program to do what you're asking, but I honestly don't think it's necessary for it to be in mIRC.
First off since mIRC uses Media Player to actually play the songs I'm reasonably sure that using the current implementation it couldn't detect anything about the recorded volume level of the file, which would mean mIRC would have to add support for mp3/ogg/wav playing internally. Secondly, I don't see a reasonable need for it. I would imagine that /splay was added to mIRC so that sounds could be used for notification of IRC related things. Granted, a lot of people use mIRC as an mp3 player, but I don't think it's really something that should be furthered if it means taking up major amounts of development time (which, if my previous assumption is correct, it would) in favour of IRC related things.