Dr Flywheel (aka Ron N7FTZ) has provided some details of the Teensy audio add-on board. This is used in the upcoming JackAl add-on board expected to be available shortly, but can almost certainly be home-brewed by savvy constructors using the Teensy 3.6 and audio add-on board.
Here is the circuit diagram for the PJRC audio module:
The CODEC part is very much the reference design provided by the part manufacturer. The support for the EEPROM and the SD card is optional.
The libraries will be compatible with any SGTL5000 or for that matter a long list of other I2S compatible CODECs (look at the source code). If you are homebrewing an audio processor on the Teensy you should consider integration of the CODEC. The libraries will support the internal A/D-D/A of the Freescale ASIC with very few components (resistors, capacitors) added.
Sample resolution of 12-bits is sufficient for the type of audio DSP that we need, though the higher bit-rate CODECs are much better. It is a matter of cost effectiveness and board space.
An alternative simpler approach
Below shows mono circuits for a lower grade Teensy, however the circuit is the same for the Teensy V3.6 with the corresponding pins.
The input circuit (mono):
Here is the output circuit (mono):