The clock signal for the ACIA is '''unknown'''. As said above it is based one the same oscillator as used for the ZN449, however, the oscillator is probably ''whatever'' MHz (?) divided by ''whatever'' (?), and its unknown if the ACIA and ZN449 clocks are using the same divider (ie. the ZN449 samples at 19.444kHz, but the ACIA may, or may not, use another frequency).
The Music Machine uses NMI interrupt, but it is not clear exactly what it uses it for.
All of the devices on The Music Machine data bus are accessible to the Amstrad within its I/O space. ACIA transactions must use 16-bit IO instructions; the converters are accesible via 8-bit IO instructions.