[Zubersoft]

MobileSheets cannot play back PDFs as audio. That would involve an incredibly complex process and error prone called Music OCR where the timing and pitch information for every note is analyzed and extracted from an image. Very few applications can handle this well (and most that do are only for Windows and cost hundreds of dollars per copy). There are no open source libraries for this that are usable in a commercial application. You can add audio files to your songs (https://youtu.be/l3R8MuKF9tY) and play those back with the audio player though. In the store listing for MobileSheets, I made sure to put this comment in it, but I'm guessing you may have missed it:

This application can not play back PDFs, images or text/chord pro files. It can only display those files and play back audio files.

Mike