If you're JailBroken, install and open up iFile from Cydia. Navigate to
/var/mobile/Applications/, tap the gear in the bottom-left corner and make sure "Application Names" is set to
On, and then find the app you mentioned inside that folder. To completely disable audio from that app, you're going to need to delete every audio file inside that folder (and subfolders). It might take you a while, but that's the only way to do what you want without writing your own JailBreak tweak. If the app you're muting is written poorly, it may crash when it cannot find those files you deleted. If this is the case, just reinstall the app, and you won't be able to mute it until somebody nice comes along and writes a tweak.