The incoming signal from the microphone amplifier is sampled to an 8-bit resolution at a rate of 19.444 thousand samples per second. This yields an analogue bandwidth of approximately 9.5KHz which is in fact the cutoff frequency of the filters.
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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 some ''whatever'' MHz (?) divided by ''whatever '' (?), and its unknown if the ACIA and ZN449 clocks are using the same divider (ie. the ZN449 uses samples at 19.444kHz, but the ACIA may , or may not , use another frequency).
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.