The prototyping area of the Feather M0 is handy for building a voltage reference (aka virtual ground) around a LM358:

I know there is a DC blocking capacitor built in the recorder audio input stage (or yet a DC-servo circuit to track the signal mean over time): I did feed the 0-3V3 directly into it (hence a signal with a 1.65V DC offset) but it resulted in ringing and more importantly in a lag of almost 500 μs ! I'll try the same measurements with an onboard cap to see if it makes any difference. I'll post data latter.