"It was used with a wind midi controller."
note the INs. Anyone know if those are audio ins or controller ins? If audio, do they replace the existing OSCs in the signal paths or are they in addition?

Also via peterwendt in the comments:
"The EXT IN jacks are audio inputs to the VCFs on two of the voices. If you plug in only one audio jack, only one voice is used. The inputs are in parallel with the VCOs, but Akai intended for you to turn the VCOs off. The main intended application is to process external sources under the control of an Akai wind controller. You can play 4 or 5 synth voices in parallel with the (2 or 1) EXT IN voices.
Full details are in chapter 10 (page 61) of the scanned manual, which you can download at this site.
This is a good site. It's one of matrix's links on the old matrixsynth site."
Wow! Someone that actually remembers my old site. :)