A quick shout out for a couple of my favourite online tools:

Icon Maker:

http://makeappicon.com/ - this is the tool I used to create all the icon files I need for iOS and Android

Audio Converter:

http://media.io - if you are going to use sounds cross platform they really need to be WAV. Media.io makes converting them from MP3's easy, and is also for converting WAVs to the right frequency/bit rate for Cocos2dx (tip: if your wav doesn't play, find one that does, and convert it to the same frequency/bits as that one using Media.io