Which sound format to use? iOS Notifications

I have 12 custom sound notifications. They are currently .wav and are 500kb per sound which I think is pretty big. Which format/codec should I use and how do I convert it? I'm trying to get the file size as small as possible but without losing any quality. This sound notifications are for an iPhone app. I tried converting it to caf with "afconvert -d LEI16 -f 'caff'" but that made the sound file even bigger.

Thanks