The top image is the original analogue circuit. I was asked to modify it so it amplifies a microphone with an input resistance of 400 ohms with the same gain. This is shown by the bottom image. Was also asked to add a second input with the same gain for another microphone. I believe i've done this by adding in the second signal generator but when i added it, the graphs measuring the voltmeters, left graph to the left voltmeter and right graph to the right voltmeter, went haywire. Just want to know if i'm right and the circuit is fine or if i'm wrong and i would like to know why and possibly how to fix it.

 

It went wrong because you changed the gain. Original circuit gain 470k/47k = 10. New circuit gain 4k/10 = 400.
Your new circuit also has an input impedance of 10Ω. Is that intentional?
Set the input resistor to whatever you want the input impedance to be and then set the feedback resistor to the desired gain multiplied by the input resistor value.

 

Your 0.1Hz signal is AC-coupled to the opamp input whereas your 500Hz signal is DC-coupled to that input.

 

It went wrong because you changed the gain. Original circuit gain 470k/47k = 10. New circuit gain 4k/10 = 400.
Your new circuit also has an input impedance of 10Ω. Is that intentional?
Set the input resistor to whatever you want the input impedance to be and then set the feedback resistor to the desired gain multiplied by the input resistor value.

Yeah this really helped a lot, i completely overlooked that i changed the gain when i changed the resistors. My input resistance was supposed be 400 ohms, not have have a gain of 400. Thanks for your reply, I've learnt my lesson.