I have two Sansa Clips. I have downloaded the Firmware Update to each of them. When I download audiobooks from NetLibrary they play on my computer. The books show as loaded into the Sansa. When I play them on the Sansa the timer goes as if they are playing, but I get No Sound.
It’s not clear what order you did everything in. I had a problem with a Netlibrary book not playing after transfer via MTP to the clip, I then tried playing it in windows media player and it worked. After I synced the netlibrary book back to the clip it did play. What apparently happened was that WMP pulled down the license file for the book when I tried playing it, so then when I transfered the book the second time WMP had the license info to send over as well.