I upgraded to Yosemite. Then I had to uninstall and downgrade to 10.8 as an app I use isn't supported by higher IOS. Now my itune files won't open. The message is "the file cannot be read because it was created by a new version of itunes.

MP3 files are MP3 files; the iTunes version does not matter. However, your iTunes library (the database that tracks all of your media) is very much related to the version of iTunes. When you upgraded to a later version of iTunes (along with the OS X upgrade), it updated your iTunes library to work with that version. By going back to an earlier version of iTunes, that older version cannot access the updated iTunes library.

When iTunes updates its library file because the iTunes application is upgraded, it may back up a copy of the old iTunes library file in the Previous iTunes Libraries folder. You can try using this procedure to restore your old iTunes library file (from immediately before the upgrade).

If you don't see your content after you update iTunes - Apple Support

Are you sure this app's developer does not have an updated version that works with later versions of OS X? It's odd that support suddenly stops at OS X version 10.8.3. It should at least work with 10.8.5 (the last update of Mountain Lion).