Hi MarkT, I'm not sure being clear with the terms: When you say "across" the supply of the SD reader, it means between 3.3V and Ground?
And "in line" means on the 3.3V line wich goes into the mic right?
Otherwise, great news: i made it work once with my arduino UNO. I changed some wires and re-started everything at the beggining, on a breadboard (before, everything was soldered)But now i try to make it work on a Feather from adafruit and the same old problem comes again, idk why...Azeez, if you're hearing me, try to do like i did, and also to use two different breadboards for the mic and the sd card reader. It might be a dumb problem like this.
Do you have a ground loop? That's not going to work with microphone-level signals, must have star-grounds.