I'd like to share my crude audio compression effect for teensy audio library:
https://github.com/newdigate/teensy-audio-multipressor

Its a very basic attempt at a compression effect

If there is any interest I would be happy to integrate into cloned teensy audio library repo and send pull request.

There is a directive in the sketch to switch between SGTL5000 and TGAPro sound card by BlackAddr.

The sketch uses two AudioFilterStateVariable filters on i2s stereo input and applies a compression effect to each of 6 filter outputs before combining outputs of each compressor back to left and right channels.

The audio queue objects are used to display oscilliscope view. you may need to comment out, until I add some extra compiler directives. I am using ST7735 display (128 x 128) to display the input and output oscilliscope view.

Please feel free to criticise, review, question, modify, extend, improve...