The dual-inverting op-amp works as mixed signals volume control (0 to n) instead of gain control (1 to n) from non-inverting op-amp. Note that the resistance values used with the op-amp voltage gain is subjected to the input impedance value (total resistance from all components ie. R1, R2, R3, VR1, VR2, VR3, ).
Note that I’ve added dual smoother-ing capacitors of 4.7 uF between +12 to Ground and -12v to Ground and dual reverse current protection Schottky diodes.
You can mixed either AC signals input (audio) or DC signals input (controlled voltage) with AC offset switch option.