My son did this for a while around age 5, but doesn't seem to anymore. With him too it was only with reading. Later he replaced the "under the breath" thing with different little "tics", like for a while he would make a very soft little grunting noise in the same sorts of places he would have previously said a word... I think for him it was some way of processing "input". He also did the grunt thing when watching TV, which again I connected to him receiving/interpreting "input". At that time I didn't know whether it was AS or tics or both, but I tried not to worry about it since it didn't seem to bother him, and it didn't bother anyone else. Unlike what many people have said about tics, he did these things when he was relaxed and enjoying himself, but actively "processing"....so I don't think it was an anxiety thing.