The tab is on the screen and it lights up when it plays but makes no sound, my speakers are working and I've tested them with videos on youtube.

I downloaded powertab (library and editor), then I downloaded the Powertab for "I've Always Thought of You" by Tommy Emanuel. I didn't really know where to save the file so I saved it in my documents and then just right-clicked and selected "open with" and then "powertab editer". I realise by doing this I bypassed the library thing. Could that be why it's not working? If so how do I get the file into the library?
I had this problem the other day, have you tried seeing if your midi synth was turned all the way up in your volume control?