Playing audio files and processing audio data from an AudioIOCallback...?

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, so that it can be mixed with audio coming from an audio input device.

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

Thanks in advance for any help you can offer!