1st put "yourmp3file".mp3 file in the raw folder(ie inside Res folder)
2nd in your code put..
Notification noti = new Notification.Builder(this)
.setSound(Uri.parse("android.resource://" + v.getContext().getPackageName() + "/" + R.raw.yourmp3file))//*see note
This is what i put inside my onClick(View v) as only "context().getPackageName()" wont work from there as it wont get any context