mIRC won't let you play 2 files from the same sound group at the same time, so you can play a wav, a midi, and an mp3 at the same time, but not 2 of each. Playing a 2nd simultaneous wav would involve playing it without going /splay, so perhaps through a dll or the /run command, at which point /splay no longer can affect it and you would need to find a way to halt those sounds.
A compromise might be to have your background song be a .mp3 or .midi, while having your action-related sound effects be .wavs.
I had trouble reading the filesize in the lower corner of your video, and I'm curious how large of a script file you're using? At some point, if you're not already long past it, you're probably going to be better served splitting your huge script into multiple script files.