It would help to know exactly how it's failing. But one thing that would probably work is this: The stock notifications and ringtones are stored in a system area inaccessible to the mortal, unrooted user. However, app notification selection dialogs will find items stored on the SD card in folders named "notifications" and "ringtones". So, you can connect the phone to your computer via USB, find the SD card in Windows Explorer, and copy your .mp3 file to the notifications folder (creating the folder first if it doesn't exist). I've found that some apps weirdly look for their notifications amongst the ringtones, so you might have to put it in both folders if the messaging app doesn't list it when it's in the notifications folder.