Default [Q] Can't save build.prop edits - Rooted MF3

Hi, first time poster here. I can usually find what I need, but I'm stumped here.

I'm running a rooted Galaxy S4 on ATT with the i337ucuamf3 build.

I've successfully installed viper4android, but it only works on and off, with Google Music.

I'm pretty sure the fix for this is to change the build.prop lines: Ipa.decode, tunnel.decode and Ipa.use-stagefright.

I've tried everything that I can think of, but I no matter how I make the changes, they will not save. I've tried mounting System R/W in Rom Toolbox, and booting into System R/W. I've made the changes with Rom Toolbox Pro and ES File Manager. Nothing works.

Can anybody tell me if I just can't make these changes to build.prop because of the build I'm running?

Please, any help would be appreciated. When viper does work, it's incredible!

Thanks.