Few people have been unable to get custom ringtones to play on Samsung a930.
Here is the fix:
Whenever you put .mp3 or .wav on your MicroSD card and change their extension to .mid, and put them on your phone, you are able to get them to play when you are selecting them as a call or alert sound. However, when you actually do get a call or alert, these sounds do not play at all.
This is because the files are still on the MicroSD card. Once they are moved off of the card and onto the phone itself, this problem is completely resolved and they play as normal.