i'll try but honestly i don't think that the mp3 have their place at the play sounds level at all, mp3 to be put on music folder is imo a fine way to separate play music than play sound sounds, and i don't see any negative impact to move them on play music, worst case, the user has only one set of speakers and both levels will be played on the same speakers
My private version of VP is set up to do exactly this. It's not difficult. I was going to post the patch, but then the sound routines started changing immediately after VP 9.2 was released. It basically just adds a second sound device to the control panel, and then sends music, or pinsounds with a special tag in the name to the backglass speakers. It's backwards compatible too - if I use a stock VP player, the sounds just go back to the cab's speakers.
I find that having the table noises at a fixed level, but the music/DMD easily adjustable with the volume buttons is fine. A real pin is that way. 
Edited by DJRobX, 20 February 2014 - 02:26 AM.