[coreyp_1]

You are correct. I need submixes for each channel. And, I mis-counted my inputs, and will nix an output, but that still leaves me with a 16x16 mess.

I've spent the weekend looking into this. I would love to build my own, but I'm not sure where to start with the FPGAs. What FPGA would you suggest that I check out for this application? I don't mind buying stuff and experimenting, I just need to know which direction to explore!

Thanks for your comments!

If I can get this to work, I'll definitely open-source my results!

[error503]

If you want to experiment with FPGA, the Lattice iCE40 series is a good starting point. It's not super capable (but will be more than enough for your project), but it was one of the first with a solid open source toolchain and has a strong open source / open hardware community as a result. It's also quite cheap.

Check out the iCEBreaker project for a decent board to get going with, and some educational materials: https://www.crowdsupply.com/1bitsquared/icebreaker-fpga