Yes, ADPCM-A samples are fixed to about 18.5 KHz. The ADPCM-B / Delta-T unit allows for a variable sample rate.
In order to get proper sample rates, I'd probably just make a vgm2txt and then search for "DELTA-T: Playback Prescale"
You can get the internal chip frequency by combining the "Prescale High/Low" values.
Code:
0x0006A55C: 5C 11 56 Y8950: DELTA-T: Playback Prescale High 0x56
0x0006A562: 5C 10 B2 Y8950: DELTA-T: Playback Prescale Low 0xB2
results in a chip frequency value of 0x56B2. ("High" = upper 2 digits/high byte, "Low" = lower 2 digits/low byte)
The effective sample rate should be (chip sample rate * frequency value / 0x10000).
So in this example (Y8950 @ 3579545 Hz, sample rate 49716 Hz), we get a sample rate of 16837 Hz
Note: The YM2610's sample rate is going to be different. I had a Y8950 file at hand, so I took that as an example, as the DeltaT unit is the same.