You can, instead of deleting the whole offline.appcache file, remove just the lines with sounds. Open offline.appcache with a text editor, and remove the lines that have at the end .ogg or .m4a.
Then, I think, it should work offline.
Another solution is to wait until Apple fixes the bug.
EDIT: It looks like the bug was/will be fixed with iOS 9, it works on iOS 9 Public Beta 1.