Unfortunately the OP Sound Card code is not directly portable to any other processor than the discontinued Teensy 3.x devices, which had very specific sound capabilities that made them unique, and which my project was developed around. To recreate the sound card on different hardware will require not only a new board but a complete rewrite of the firmware based on a completely different approach. This is not something I'm going to undertake, but user Foxhood recently started a thread about his project which he calls OPSC Lite, which you should follow.
I would note that the code is like 98% compatible with the newer Teensy 4.x as though the main website doesn't indicate as such: the sound library has been ported over cleanly to the new processor, restoring all functionality except for the on-chip DAC as the T4 lacks one. Porting would mostly be a matter of a little bit of rewiring (SD from SPI to SDIO), adding a simple little I2S DAC (e.g. SGTL5000) to the board and a few relatively tiny tweaks to the code like swapping the Audio DAC output block with I2S Output and replacing the old SdFat fork with the regular version.I investigated this possibility as part of my dive into potential solutions to the audio dilemma. Honestly would have gone for this approach if my experiments didn't show a very interesting possibility in a spiritual successor to Benedini's work with AVR. Still. I'm keeping this in mind as a back-up should my efforts fail to pan out. After all. We do need a solution