OP1A is the inverting summing amplifier with a gain of unity (A = -RCR 22K / Rin 22K = -1, or 0 dB).
OP1B is the output stage with a gain of 10 dB (20 log 47K/15K), or 20 log3. It is also an inverting amplifier. Therefore, the input and output phases are maintained.
A few 10µF and 22µF capacitors provide coupling in the audio path. Two 100nF capacitors decouple the NJM4558L integrated circuit.
A little tip: this circuit can be built using a SIP8 vs. SOIC8 adapter, which I published on Elektor Labs:

https://www.elektormagazine.com/labs/sip8-vs-soic8
Although I have already built stereo mixers, this mono circuit would be suitable for pre- or post-fader auxiliary inputs. mix of effects or intercom feedback.

EXAMPLE: 4 INPUTS

MONO  REDUCER:
 With a few modifications, my module can become a stereo-to-mono signal reducer.
The output has been changed to a pseudo-balanced XLR connection.