Building an Audio Analyzer with Matrices and Mega2560 / Uno and FFT and Op amps

Not possible.

If there is no LM386 with open loop configuration, than you shouldn't bring this argument (open/ close loop ) in discussion. It's not related. I don't know what is the definition for infinity:

An ideal op-amp is usually considered to have the following properties, and they are considered to hold for all input voltages:
Infinite open-loop gain (when doing theoretical analysis, a limit may be taken as open loop gain AOL goes to infinity).

IMHO, infinity is starting from 10^5.