This Audio Board is and extension for your Teensy. It exists for Teensy 4 and Teensy 3. If you want to use it you have to solder it together with your Teensy. It has an anormous amount of useful features, including for example high quality audio adapters in 16 bit and 44.1 kHz sample rate. It supports stereo headphone and stereo line-level output, and also stereo line-level input or mono microphone input.
Teensy Audio Library
This is kind of a software you can open in your browser to program your signal flow on the Audio Board to create all types of sophisticated audio applications. You can play multiple sound files, create synthesized waveforms, apply effects, mix multiple streams and output high quality audio to the headphones or line out pins. To use the sketch with your board you can export the code and apply it to your Teensyduino Software and upload all of it.