Thanks for logging this project Paul. I decided to make a Teensy based sequencer for my children this weekend. As in, I made the decision in a weekend (I know nothing about audio so it took me a long time to work out what I was trying to make for them). It will take me a very long time to build it. 

Looking forward to seeing how this project of yours turns out but I think I will stick to the audio shield! 

By the way, have you seen the high density memory chips recently tweeted by Greg Davill and tested by adafruit? Wondered if they're a good fit for teensy audio https://lcsc.com/product-detail/FLASH_XTX-XTSD04GLGEAG_C558839.html

Maybe check out the Monolith Synth project.  4 of us built it in 5 weekends.  It had a 40 arcade button sequencer on the front and a lot of other synth stuff on the sides.

I saw Greg's message about those chips, and also Adafruit's.  Most SD cards are fairly high random access latency which is less than ideal for playing more than 1 sound at a time.  Will be interesting to see if those chips are any better.  But my expectation is they'll be about the same as cheap SD cards.

Lately there's been a lot of development around filesystems and storage.  All the discussion is on the forum.  We're also in beta testing on new software, again on the forum.  That's the place to go if you want to get the latest developments.

Checked out the monolith - what a cool device! I will head over to the forum.