With a slight modification to this other method of injecting javascript (https://www.axure.com/c/forum/tips-tricks-examples/20739-improved-javascript-injection.html) --I was able to get this responsiveVoice to work in Axure.
See details in that thread, and here is a test rp file that works for me in Chrome and Firefox (but not Edge or MSIE).Javascript with External JS.rp (58.4 KB)