I got my M8 yesterday and have root with S-ON. (The link says on did this with S-ON) I've have renamed the mp3-file and also tried to edit the default.xml file.
In both cases the phone reboot istself and reverts any change I have made.
So I beleive I need S-OFF, even one said in the link he did this mod with S-ON?
I forgot if SOFF is required; but, you need to remount RW; so, either do
http://forum.xda-developers.com/showthread.php?t=2701816
or, make the change from recovery.
If phone reboots itself, it's an other issue. If just the module was missing, then, then change would be canceled after reboot, but after manual reboot.
Auto-reboot on write means you have an other issue. For me, it was that the file system was corrupted. I needed to run fsck on /system; it was not included in my ROM. A friend build it for me. Fsck found errors, fixed them, and them, i had no more reboot on write.
I will upload fsck on the thread about the module in a few days.