Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
SunVox Updated (06/26/2014)
What's New in Version 1.7.5
64bit CPU support;
SunVox file type association: now any SunVox-compatible documents (sunvox, midi, xm, mod) can be opened from other apps (Mail, Browser, etc.);
new module - Pitch Shifter;
customizable curve of the values has been added to MultiCtl module;
MIDI Octave Offset option has been added to the MIDI Preferences;
PR (Set note period) command has been renamed to SP (Set pitch);
main menu has been divided into two separate menus: the Main Menu (left side) and the Pattern Menu (right side);
UI design slightly changed;
new simple song examples: crossfader;
new song examples: Twistboy - Translucent, unspoken_words;
Was pleased to see this update for this deceptively deep and powerful app. I had for weeks and didn't get it at all. Finally played around with it and realized one by one: "Holy crap, it can do ____ as an AB effect"; "Holy crap, you can make a ton of synth sounds with this thing"; "Holy crap, there's a huge library of free samples you can import into this thing". And I still only understand about 2% of what it can do.
The free PC version is arguably easier to use because of the interface, and is a great way to get familiar with the app. Amazing developer and project, however.