Non-inverting Op-Amp Audio Mixer
I am summing 3 signals from sources and only 1 is generating sinusoidal signal with 100mV amplitude I get 33mV at the output of non-inverting configuration and in inverting case I have 100mV at the output. Why?


In the top amplifier the summing junction is a virtual ground so the feedback current exactly matches the current into the virtual ground and the output is the inverted sum of the input voltages (modified by the resistor ratio, - here unmodified because all the resistors are the same)

In the bottom amplifier the 470K does basically nothing, so I'm going to ignore it,

The three inputs flow into a three-way voltage divider so given that the resistors are equal they get averaged  so 0,0,100mV  gets you 33.3mV 

the amplifier just buffers that gives a 33.3mV output.

If you want 100mV from the non-inverting amplifier configure it to have a gain of 3.
eg: 2K from output to inverting input and 1K from there to ground.