Just a brief note: It turns out there's a whole body of research into the question of whether listening to text while reading it improves various measures of reading performance. (If you search for "listening-while-reading" on Google Scholar, you'll find a large number of papers, many behind paywalls.) Some seem particularly relevant to the questions raised above; e.g.:
Shany, M. T., & Biemiller, T. (1995). Assisted reading practice: Effects on performance for poor readers in Grades 3 and 4. Reading Research Quarterly, 30, 382-395. doi: 10.2307/747622
Examined effects of assisted reading practice over a 4-month period with at-risk 3rd- and 4th-grade children. 10 subjects read basic materials orally and were assisted with word identification by a teacher. Nine subjects read while listening to a tape recorder whose speed they could control. Assisted practice significantly improved text reading rates and reading comprehension scores of both experimental groups compared to a control group, although gains in letter-naming speed, decoding, and reading speed for words out of text did not reach statistical significance. Listening while reading resulted in twice the amount of reading as the other method and led to higher scores on listening comprehension measures. Gains in reading comprehension were larger when there was a large pretreatment difference between listening comprehension and reading comprehension.