I’m trying to do something similar-ish as well only by trying to mess with the hardware rather than using the audio engine.
In my case I need a variable sample rate as I’m wanting to simulate how the GameBoy output audio on it’s WAVE channel. The GB used a timer that would be adjusted according to the desired pitch. The timer corresponded to the when to switch to the next sample in the WAVE. The clever thing about doing it this way is I don’t have to worry about interpolation. As long as the SPI bus is running fast enough, the precision lost is minimal and it’s appropriately chippy. You can read more up on what I’m trying to do here.
So far, I’ve been thwarted since the Daisy likes to use fixed sample rates for the Audio DAC as well as the CV OUTs (I’m building against the Daisy Patch MS). Technically I’m sure it’s possible to get around both but I’m currently trying the CV route since it seems like it might be easier to do. It does have me wondering about perhaps switching to a Teensy plus an SPI DAC since I’m it seems like I’m having to circumvent much of what makes the Daisy awesome.
While I was digging around the forums, I recall reading one of the devs said they are working on arbitrary sample rates at least in between 8Khz and 96Khz so that might be something to follow (I don’t see any support for that in the lib documentation yet though). This would be WONDERFUL though so I’m hoping we see it.
Not sure if any of that is helpful to you or not but I tried 
I think were you not to do any of the above, you’re going to be stuck where I was, which is to resample and interpolate your wave up to whatever the current (fixed) sample rate of the DAC is set to.