Absolutely Essential for Musicians

The first and still by far the most reliable and time-tested method to route audio between apps. Drop dead easy to use. You want to make music on iOS? Buy this app. Then stop by their forum. A lot of iOS musicians there who can help out with just about any question you might have.

Audiobus - the gift that just keeps giving

Sturgeons Law about Audiobus 2, v2.3.8