Mix audio file playback with audio device input, and play to audio device?

I’m writing an audio application that is primarily a “mixer”. After asking some questions here, and doing some experimentation, I have gotten a pretty good handle on how to open audio input/output devices, and process the audio in an AudioIOCallback.

Now… I would like to add a feature where I can play back 1 or more audio files, while playing/processing the audio coming in from the Audio Input device.

I can not find an example that seems clear (to me) about how to read audio files (from within an AudioIOCallback), in such a ways that the the audio from file can be mixed with input from an audio device, and played to the output audio device.

Any ideas where I can find any example code on how to to this, or any (clear) documentation on how to do this?

Thanks in advance for any help you can offer!