Changes

The SP0256 is (usually) driven by a 3.12MHz oscillator, and it uses 7bit PWM output. So the sample rate should be , which is clocked at 3.12MHz/2, to obtain a 10kHz sample rate, the chip issues some dummy steps with constant LOW level additionally to the 128steps needed for 7bit PWM, probably further divided by two:making it a total number of 156 steps per sample.

Sample Rate = 3.12MHz/256 2/156 = 1210.1875kHz 0kHz ;82.051ns 100us per sample

Which means one sample is 82.051ns 100us long, that value multiplied by 64 or 91 gives the following timings per repeat:

56.251ms 4ms per repeat (noise and pause), or 79.466ms 1ms per repeat (tone with pitch=91) That (guessed/undocumented) samplerate does more or less match the specifications that say that (unwanted PWM-) noise is above 10kHz, and that wanted pitch can be up to 5kHz. And the repeat timings do more or less match the timings shown in the allophone list.