It seems that the Read Aloud tool advances the resource text along with the reading voice only at a reading speed of 1.0x. When reading at 1.4, 2.0 and 0.75, the reading voice continues beyond what is shown on the screen and the text does not advance. When I scroll down to keep up, it causes the reading to stop and re-set.
I don't know if this behavior is new to Logos 8 or existed in Logos 7. I've only just now started using the Read Aloud tool.