I have occasionally encountered this problem and not just with NVDA as the screen reader. The alt-tab solution will certainly work. However, what usually works for me is to press the alt key twice. The first press will move focus to the ribbon and the second press returns focus to the document window and speech should behave as expected. The bottom line is that you want to move focus away from and then back to the document window. Alt-tab certainly does this but pressing either the alt or the Windows key twice will do the same thing and may be a tad easier and quicker.