I have a WAV of R2D2 that I use for my Text Messages notification sound. I put this file in a folder on my phone called "Ringtones" - along with my phone ringtones, as I did on previous versions of Android.
And yet, when I go to change the notification sound, the R2D2 sound effect is not available to choose.
I moved the WAV into the NOTIFICATIONS folder on my phone, and viola, it's on the list now and everytime I get a text message, R2D2 beeps at me. Like the Star Wars nerd I am.
Maybe this was obvious to everyone else, but it drove me nuts for weeks recently until I sat down and fixed it today.