Distorted playback

Just having a little trouble with the playback. I grabbed a copy of Nocturne off the web site and found that the playback was terribly distorted so I tried another and had the same problem. So I downloaded the Fluid synth and set that up but had the same problem. I then tried experimenting with different volume level combinations but had no joy there either. I have no trouble with Sibelius, Encore, or playing the samples so it is a bit of a mystery to me. Any ideas how I might fix this? It's bound to be something simple.

thanks
Bryan