First, if you have two soundcards, they probably conflict with each other. That happened to me too, i had an OnBoard sound card plus a sound blaster. My Onboard didn't conflict with my SB, but sometimes, mIRC would screw up for no reasons. After i disabled the onboard sound in the bios, it all ran smoothly afterwards. The answer is no, you can't specify which sound card you want to use with mIRC. You can however specify Windows' default sound card.
Can you hear you music with other applications such as Winamp or WMP? If so, try disabling a sound card, then put a mp3 in your mirc directory and type //splay $+(",$mircdiryourfile.mp3,") and tell us what happened.
There is no error or so, but I cannot hear something. Mhh...