Connectors on the sound board or possibly a ROM issue. If you have not done so already, check the condition of the pins and the solder pads for the pins on the underside of the sound board. If the solder is not shiny, you should reflow solder over the pads. Also check your connectors, especially the ribbon cable between the sound board and CPU, and make sure the wires are crimped in place snugly for non-ribbon connectors. If you still have issues, it could be a component on the sound board related to the speech ROM, or the ROM itself. Usually when the ROMs have issues though, you will get garbled output/artifacts or nothing at all permanently, not intermittently, so I wouldn't assume that it is bad until you try everything else first.