I copied the mp3 (kalimba.mp3) file into the raw folder in the res folder. But when the notification is triggered it produces the default sound.
raw
res
R.raw.kalimba is an integer resource ID; you want the name of the sound resource in that Uri. So try:
R.raw.kalimba
Uri
notification.sound = Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE + "://" + getPackageName() + "/raw/kalimba");