archive.worldofdragon.org

Bad luck with the 16K, but there are an awful lot of them out there. At least you didn't get an ultra-early 4K model. ;) (Or maybe they're worth more?) Upgrading might be possible... I use this site whenever I see a CoCo on eBay, it has details of a fair number of specifications for various serial n...

The nearest I can think of is Jon Bird's add-on soundchip schematic: SAA-1099 Schematic I think he takes the stereo output to two headphone sockets, but the cartridge port does have a line marked "Ext Sound In" which may do what it actually suggests... Potentially it could be used for any mono sound...