The issue you may be experiencing could have to do with a quality issue on the 3.5mm jack (item 1). The switch on that batch of jacks goes bad often, meaning the 1/4" input and microphone doesn’t work at all until something is patched directly to the decoder input. It can be replaced with this part pretty easily, and we’re happy to do it here if need be: https://www.mouser.com/ProductDetail/Kobiconn/16PJ138?qs=oQGaBZOgWEbaXSfkKZSXCA%3D%3D
Well, I don’t appear to be having the problem you mention. I don’t know what errors I made last time I tried to test it (perhaps I had the gain turned down, I dunno), but it’s working perfectly now. At least this issue is now documented.