CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
JN Soundboard - An open-source hotkey soundboard program
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Soundplant - Play sound files with the computer keyboard
FL Studio Mobile - Create & save multi-track music projects on a smartphone