Would love to be able to add my own MP3 file to the video. Did a nice little impro on Piano, sent it to my phone and alas, not going to happen tonight. Guess will have to work with Premier Pro tomorrow.
You should be able to add your own MP3 from the mobile app. When you click the + button, make sure that you select Your Media (as opposed to Audio which contains audio that comes with Premiere Rush) and navigate to the MP3's location.