speech reading is something that requires practice. and some people are better at it than others, thus why some deafies are able to pass off as hearing to the hearing population, and others are not.
i personally dont try to 'dub' a voice in with the speech. I just look for the mouth shape and possible other cues such as body language to determine what the speaker is saying. This obviously isnt 100% effective as some people are just near impossible to speech read while others are easier.
