Pins 3 and 4 of the LM386 are both connected to 0V (the negative side of the battery) so you shouldn't measure 1.31V on pin 4. It means you have no connection to pin 4 for some reason. Are you sure the 1.31V isn't on pin 1 rather than pin 4?
The Ruby 2 diagram has a serious error, the input jack connections are reversed. You only need screened cable for the signal input to the jack and between the jack and the circuit board but the screen is the outer braided layer of the cable and it should be connected to the 0V side. On a jack plug the 'body' cylinder of the pin is ground and the tip carries the signal, on the socket, the pin joined to the ring around the hole is ground and the furthest contact is the signal. When the plug is inserted, the screen on the plug connects to the ground side of the socket and the signal wire connects to the signal wire. When it is reversed you will pick up interference from the cable, you would have the screen signal being amplified instead of the signal you are trying to protect.
Brian.
The LED is connected backwards and the value of R4 to limit its current is wrong.
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