I found an 8 channel audio processor IC from TI that can do mixer routing from SD I2S lines. I'm thinking of doubling each mono I2S input and allowing PAN between the stereo pairs 1/2 3/4 5/6 7/8 then having 1,3,5,7 channel output to LEFT SDout ( I2S ) and 2,4,6,8 channel output to RIGHT SDout ( I2S )

but, I'd need 3 TAS5558's to do digital mixing for 8 - 2 channels and possibly a final 2 or 4 channel DAC to output the 2 or 4 channels of analogue balanced audio.

Also looking the FPGA or DSP route but have no idea how to setup simple audio channel routing and mixing this way without writing channel mixing coefficients from scratch ! and I am a total beginner at all this !!! if anyone can point me in the right direction for audio mixing in FPGA or DSP I would much appreciate it !!