Inaccurate voltage readings on an esp32 as opposed to multimeter
0.1 Hz is not enough to measure audible frequencies. You need to get at least double the sampling frequency for the signal you measure. See also: https://en.wikipedia.org/wiki/Nyquist_frequency
Say you have 500 Hz, then you need to sample at least 1000 Hz, or 1 ms.
Then you can attempt peak detection.