Summing multiple audio signals while keeping a copy of the originals
The summation should be easy enough, just use an Opamp Summer (no, not the season!). You'll just need to extend the design to 6 inputs, rather than the 3 in the example.







simulate this circuit &ndash; Schematic created using CircuitLab

And the output is described by:
$$V_{Out} = V_1(\frac{-R_F}{R_1})+V_2(\frac{-R_F}{R_2})+...+V_6(\frac{-R_F}{R_6})$$

The only major limitation here is the output will be inverted -- you can fix this by putting another inverting op-amp stage after this.

As for the DC Bias required by the ADC, you can do this with a buffer stage as well:







simulate this circuit

Typically, R1=R2 so your DC bias will be half of V1, and C1 is calculated to be large enough for your lowest frequency of interest: $$f_c=\frac{1}{2\pi R_1||R_2 C_1}$$This implementation also has the advantage of allowing you to run it from a single voltage source, rather than a split supply.