1)Most uprocessors are driven from 32.768 kHz, and are integer multiples of this frequency,
2)1.5728 MHz is the lowest frequency, maybe it could be done using a custom crystal or by tuning the crystal...

1) e.g. the f12x has 5 timers, 3 of which are T2 type. with that resolution and a high internal clock frequency (will run at 100 or 50MHz) there is no need for a "multiple of" crystal.
2) It would surprise me enomously if CODEC crystals were difficult to find.

Erik