The LUA api includes:
playdate.sound.twopolefilter:setMix(level)
playdate.sound.twopolefilter:setMixMod(signal)
In addition to frequency and resonance. However the C API only includes the last 2: Inside Playdate with C
The same appears true for all of the SoundEffect types, such as RingModulator, Bitcrusher, etc.