I don't think there is a way to start in the middle of the page when using the voices on the Web Reader. My workaround is to have the students still select "read now" to open the book in the Bookshare Web Reader and then use the text-to-speech tools in the Read&Write for Google Chrome extension. (We have licenses of the premium version, but Text-to-speech tools are available in the FREE version!) The student can highlight the text and select the play icon or use "hover speech", which allows the student to put their cursor in the location that they would like to begin listening. However, we have found that hover speech can be a bit quirky on the Web Reader at times.
Hope this helps!