Quoted from 320Gigabytes:

Yeah, this is all from memory so it might not be 100% correct, anyone else can chime in and help out.
But just open the IM file on pinball browser.
Go to your audio section.
I would go through the list of audio files, and click on the ones with the longest time. These are probably your general themes for the game/modes. Preview them each to find the one you want to replace.
Find the theme you want to replace with Shoot to Thrill. Write down the EXACT LENGTH of the audio file.
Now, open "Shoot to Thrill" in an audio editing program (Audacity is what I use).
Then change the audio track to mono (Tracks>Stereo track to Mono), and trim the song to fit the exact length you wrote down.
Go to File>Edit Metadata and clear all the metadata associated with the track.
Then export the song in a WAV format.
Go back to pinball browser, and when you have the correct audio file selected, replace it with the one you just made.
To test and see if you have successfully replaced it, press preview or play or whatever when you have the audio selected and see if it worked. Then you can save it and install it like a regular software update. It will say it failed, but it didn't. Just don't select anything, turn it off, pull your USB, flip the proper switch back, and test it out!