PianoDisc also makes a 'front-end' that reads simple MIDI files, and will automatically convert other voices into a piano voice if that's what you'd like. That would save you from having to edit the MIDI files yourself, and convert them to PianoDiscs proprietary language. Since 'pre-conversion' MIDI files are so tiny, you would then be able to put tens of thousands of songs onto a single CD.
For the time being however, here is an example of how to edit a MIDI file...
1) Find the file your looking for!

For instance, a quick google search found a MIDI version of 'Night on Bald Mountain' on this page:
http://www.sternton.com/midi/classic.htm2) Open the file in a program that will allow MIDI file editing. You should be able to see the list of voices used.

3) Simply click on each non-percussion voice, and change it to 'acoustic grand piano'

4) The new file can then be converted using the mid2pianocd program.
Remember, even though many online MIDI files are free, they are copyrighted. Converting them to another format, and selling them for profit, is likely not a permitted use.
Actual piano audio files (such as those on youtube) cannot be made to play your PianoDisc system, since converting audio files to MIDI files is still very much, a 'developing' technology.