Howdy and congrats on wanting to go down the custom avenue.....

From all my research, i've found a LOT of dead ends with regards to audio/speech rom info, file formats, size/mapping within the rom etc. (not that there many sites or forums openly discussing audio-rom mapping & info anyway).

I gave up, and developed my own addon speech board which co-exists with the original roms, allowing you to replace ALL sounds with my board, or choose a mix of "original" AND "new" sounds etc.
(some people might find only 1 or 2 effects or speech to be overly annoying and replace it with something else, or mute it entirely).

At this stage, my board isn't completed or ready for sale as i recently had a new brainstorm for expansion, so it's being redeveloped (slowly, with interruptions called work & life).

The recent 'brainstorm' also unleashed a new development, being a controller to replace the DMD graphics with your own custom stuff, very similar principle to my FX/Speech board. (DMD graphics/rom info also being HARD to obtain).

When both these boards are upto scratch, it'll unleash a new level of hobbyists and custom pins that can finally come to life !

MM.