The usual reason the cheap electrets sound bad is that their internal preamp is run on too low a voltage. I got some capsules intended for hearing aid use (I needed very small), which cost about 50 cents each because I bought 20 of them (I think I still have one or two left). They were ruler flat, could have been used as calibration mics, but distorted early in their original circuit because it ran on a 1.2 volt mercury cell. Their specification said they could accept more voltage, and with 12 volts derived from 48 volt phantom they could accept 113dB levels before distortion became disturbing.
But the inside of an accordion doesn't sound very nice at all; all clicky bits, and nasty sharp harmonics and bellows noise and wind. I find a bit of HF rolloff, rather than accurate reproduction, to be easier to work with (accordions were never made to be listened to from inside).