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!

Sorry to bump into this but I am trying to figure the same thing out. Any improvement?