Hello and welcome to iStripper @SoxMike
You can add .mp3 of your choice in c:\users\your user\AppData\local\vghd\Data\musics and after that, go to the Music tab of the software by clicking the little icon "musical note" at the upper right of the GUI to select/unselect the mp3 you want to play with the shows. Don't forget to go in the setup (the cogwheel at the upper right of the GUI), enable advanced settings at the top, scroll down to the volume controls and make your setup here.
I recommand you to read the help by pressing the "i" icon with a circle at the upper right of the GUI.
Have fun with iStripper.