Everything that's pure DSP, all the functions that aren't hardware inputs or outputs, work fine on Teensy 4.0 and run much faster, so you can do much more. A couple of them with hardware-specific memory limits, like the delay effect, got a minor update. Use
1.49-beta3 for the latest code.
I2S (stereo) and TDM master mode input & output work great. The
Rev D Audio Shield is the best supported hardware. It works very well.
I2S slave mode might work, but I'm not sure.
I believe PT8211 works, but it's on my list to check soon.
This PCB arrived from OSH Park just yesterday.
USB and ADC and PWM are not yet working on Teensy 4.0. These are on my list to do, but I can't promise a time frame.
I2S quad channel isn't working yet. Neither is 6 or 8 channel, which aren't even possible on Teensy 3.x. I worked with this just a couple days ago, but didn't get it running yet. If you want more than stereo, for now plan on using TDM.
Teensy 4.0 doesn't have the 12 bit DACs of Teensy 3.x, so the DAC output will never be supported.
ADAT hasn't been tested. Unless someone with this hardware steps up, it seems unlikely we'll ever get this niche application well supported on Teensy 4.0.
While not technically part of the audio library, USB MIDI was recently added in 1.49-beta2. Many people use this together with the audio lib. It's working now, if you use the latest beta. Of course, regular serial MIDI and USB host MIDI have been working on Teensy 4.0 since its release in August.