This is an excellent question and it is a shame that nothing has been said. I apologise that I cannot comment directly about the speach synthesisers. I expect that this is highly unusual. However, I did want to offer a quick word of moral support - I have had a client who insisted upon using VERY LARGE BOLD font on his OneWorld Forms. When questioned on the matter he revealed that many of his operators were visually impaired...... So it is not as unusual as we might think.
Maybe others can share their experiences of using OneWorld with user with various other impairments or special circumstances? I am personally keen on touch screen technologies in shop floor environments.