A Quick REF on custom sounds in Androids.
mp3 files has to be stored in:
/SDCARD/media/audio/
And for different types, the DIR are:
/SDCARD/media/audio/ringtones
/SDCARD/media/audio/alarms
/SDCARD/media/audio/notifications
Links:
This Android Life – Guide – Add custom ringtones and notifications to your Android device.