When Stern came out with the major software update a while back it invalidated the music swap-out process that had been standardized here on Pinside because the indexing to all of the sound clips was changed. The music in the update was better than the original and it seems that no one ever went back and did all the leg work to make this mod easy again.
This is not my expertise or I would do it. Basically someone needs to go through the entire ROM in Pinball Browser and find all of the songs and write down the call-out numbers along with the song titles and the exact length of the songs. Then it is a matter of picking songs to replace them, putting the songs into the correct format and clipping each one the the correct length of the song it is replacing. Once the sound files are ready they can easily be dropped into the ROM image and a custom image can be saved. If Stern makes minor changes to the software, the songs can easily be dropped in again using the updated ROM image and a new custom image can be saved.
As I mentioned above the problem here came when Stern basically did a full overhaul of the software for this game (and made it a lot better).