Looks like you will be sampling at 10,000/second. (maybe 8,000)
Do you really have microphones that work at 5hz?
Do you want 8 bit or 10 bit data?
Last time I looked at Arduino it could sample at 10,000/second. (9,600)
Arduino to PC; You will probably need two character/data X 10,000/second. (Serial port?)
Because of noise; I would build 40 preamps, then mux 40:1. If the mux is before one pre-amp I think the noise will be too much.
The pre-amp could have a band with of 2hz to 20hz. (not 20khz) Be removing the high frequencies and (60/50hz) you could remove much of the noise.
How strong is the signal from the microphones? How much gain?