Feasibility ultrasonic recorder

Thanks Mark.
Not sure I fully understand.

  1. The mems mic has pdm output, from 4th order sigma Delta. Is all the filtering you've quoted in addition to that ?

2.why will phone reject 20+kHz ?
Eg the fft spectrum analyser app spectroid gives an option to.sample at 96kHz......

3 what platform would you recommend for such a project with say 30kHz limit ?

Many. thanks.
PS in OP I said read from the audio channel, I meant read from the mic channel obviously.