If you're willing to hack and deal with unreleased bleeding edge development, the migration_to_chibios16 branch contains code to transmit/receive digital audio over SPI in packets of 16 sample intervals on the X3 pinheader, using the
jt/link/audio object. You will need to compile everything yourself, use "dfu flashing" to flash firmware, and be ready to dig out a logic analyzer or oscilloscope for diagnostics. Easiest if your device takes the SPI slave role, and Axoloti master. 8bit Arduinos won't be able to handle the bitrate (nor add any useful audio processing capacity to Axoloti). The "GUI-refactoring" branch currently still needs further (ongoing) development to get into a usable state, so do not try that yet.