Sound should not be compromised at all if you have put a check mark in the box to retain HD Audio for BD 25 encoding (in Settings). Or did you make a BD-5 or BD-9 backup?
Movie only takes out the menu and extras. Full backup should work normally on a 25gb backup. I emphasize should, because I always make movie only backups, and haven't got much experience making full ones....not since the early days of the program.