I've designed and received a custom MEMS microphone amplifier PCB, but am having issues with poor high frequency response and very low sensitivity/gain. Basically the microphone stops picking up sounds above ~300-400 hz. These are output using a tone generator on a medium-sized speaker system. Playing loud music results in a very small response, and only at lower frequencies. I have tried 5 of these mic boards and all have the same issue, so it's not just a single faulty board.
The circuit diagram is below. It is based off of and essentially identical to the Sparkfun MEMS board. I have tried one of these Sparkfun boards on the same system, power supply, etc, and it works fine, so the issue is with the boards I've received.
Could anyone provide advice on what might be the most likely issue, which components might be trouble, parts of the circuit to test and diagnose, how to diagnose, or any other general ideas? I have a multimeter and can get access to a Oscope if needed. Thank you.