Can I change app-specific sound files? Specifically, I'm using Zello and want to change the notification sounds to make them work better with my bluetooth headset, but potential answers probably will be applicable in many cases.

Using Dexplorer, I was able to locate the specific *.wav file inside the internal folder structure of the app. Dexplorer however doesn't seem to offer an option to replace these files. Is there any way I can achieve this, meaning replacing the current *.wav file with a file of my choice?
Dexplorer opens installed APK file and displays info from APK. This is internal application resources, build when autor compiles APK.

To replace specific resources you must decompile APK file, repleace files and compile (build) again.

On new build you must sign APK file with your own certificate (you may generate it). Only author have original keystore to build signed APK file.

App author probably won't provide his application signing key.