I don't know what you mean by virtual earth mixer? Are you talking about using an op-amp as a rail splitter?
I read about it here https://sound-au.com/articles/audio-mixing.htm#s3
That's got nothing to do with whether the circuit is operating from a single supply rail, a bipolar supply or a virtual ground. Both of the circuits I posted use the same virtual earth principle, as described in that article. The difference is that first one I posted was AC coupled and the second one DC coupled. In the AC coupled design, the DC is blocked, so only the signal matters, the fact that the negative input is fixed at half the supply voltage vs 0V doesn't matter.
The circuit works by fixing the inverting op-amp input at a constant DC voltage. In most cases it's the same as 0V, because it's what the non-inverting input is connected to, hence virtual earth, but it doesn't really matter. Take the input resistors away and you'll see that the impedance into the inverting input is very low, due to negative feedback. If a current is injected into the input, the op-amp's output will move in the opposite direction, pulling current through the feedback resistor to counteract the change in voltage due to the input current.
The circuit is a transimpedance amplifier. It converts a constant current into a constant voltage. In this instance, with a feedback resistor of 10k, 1mA of current in, will give -10V out, because the input will stay at 0V and 1mA of curenet will give a drop of 10V across the resistor.
https://en.wikipedia.org/wiki/Transimpedance_amplifierKirchoff's current law states that the currents through all the resistors add together, at the node where they're all connected. Suppose you have 5V at two inputs, with 10k input resistors. The current through each resistor will be 0.5mA, giving 1mA in total, which also flows through the feedback resistor, giving an output of -10V.