Basic circuit to turn on LED when input detects signal (for guitar)
I would think an envelope follower would do what you want. Basically, rectify the signal so you are only dealing with positive voltage, then low pass filter that a little to smooth it out, and feed that into comparator to select the level you want the comparator to switch at. Drive an LED from the comparator if it has enough current capability to do so, otherwise drive an open drain MOSFET to sink current thru an LED to turn it on.