There are a few solutions for this. I'll overview some of them, trying to weight pros and cons for each one.
Built-in mics and speakers
The app should work out-of-the-box by using iPad's built-in microphone and speakers. As positive point, this is the cheapest option. As negative point, the built-in mic and speakers are poor and you'll suffer with audio feedback (ie. no overdub would be possible).
External USB audio interface
That's the option I use, with my Scarlett FocusRite 2i2. Here, you need to connect the audio interface to the iPad via a USB-lightning adaptor. This approach works fine. However, you will need to buy the adaptor from Apple (around 50 bucks), in addition to the interface itself (the i2i costs around 100 bucks). Definitely not ideal due to cost, but reliable.
The second solution relies on commercial audio interfaces for iOS such as the iRig to do the job. This particular one does not use the lightning input, but the audio jack instead. This leaves the lightning input free for charging.
This one also solves the problem, seems reliable, and it's cheaper (the cost here is around 50 bucks). I'm curious to see how this compares in terms of sound quality to my FocusRite 2i2 setup.
PS: There are some 'fake' iRig interfaces available on amazon, which are even cheaper. Try at your own risk!
Of course, build your own iRig! Some tutorials I've found are:
- Recent post about this here.
While this might be the cheapest option, chances are this 'cheapness' can directly affect sound quality. Never tested this. Try at your own risk too!
I'll use a small battery to keep the whole setup portable.
... the iPad
If you are using the iRig 2 or the DIR iRig, you can connect the iPad directly to the battery using the lightning input.
If you are using the External USB audio interface, you can use a Lightning to USB 3 Camera Adapter (ie. there will be an additional lightning input to connect to the battery).
... the Audio interface
Directly to the battery and the iPad, using a Y cable.
... the DIY circuit
Via USB Micro-B Cable - 6" directly to the battery.
Hopefully, that's it! ;)