The fact that the inverting input to the amplifier is virtually at earth potential provides some significant possibilities for audio mixer applications.
This form of virtual earth mixer, or summing amplifier adds several different signals in a linear or additive fashion. It is not the form of multiplier mixer used in RF circuits where a non-linear action is used
There is almost no limit to the number of inputs that can be added. The gain for each leg being R2 / R1y where R1 y is the input resistor for that particular channel.