Hyperion's H2 port uses cdda.library to play CDDA on old style analogue CD-ROMs. There is a MorphOS replacement for this library - check on Aminet.

The smacker codec built into the game is broken so I don't think you'll ever be able to get videos to play on MorphOS.

Heretic 2 doesn't allow you to define custom screenmodes. The best you can do, possibly, is make a screenmode of your own at 960V so that it fills the whole screen properly.

Time has not been kind on what was one of Hyperion's less broken ports...