You're meant to change the sound with your own define statement, not in the backend file! For example,
define myconfig.ACHIEVEMENT_SOUND = "audio/interface/ach.ogg"
The achievements.rpy file has examples of that. If you're only changing it in the backend, then the redeclaration in achievements.rpy will be overwriting it with None. The backend file has an early initialization order so it can be set up early before it's used and modified with a regular define statement.
So, in short, change it in achievements.rpy not in the backend file :) Hope that helps!