Custom iOS push notification sound

You can't play .mp3-files. However, you can convert them into .wav files here: http://media.io/ (online & free). Then put the converted file in your XCode project and you can call it by specifying "sound":"sound.wav"

According to the push notification guide:

You can package the audio data in an aiff, wav, or caf file. Then, in Xcode, add the sound file to your project as a nonlocalized resource of the application bundle.

It doesn't say anything about MP3s. I've used aif files which worked. I'd stick to what Apple specifies.