Can't hear playback

[chevyfiftynine]

Hi Forum, just joined tonight. I've been using Sibelius 4 for a few years and it has been absolutely great, but when I started arranging a new song for my sax class, I couldn't hear any sound either with the individual notes or playback. I went to other songs and could hear them. I noticed previous songs were set to Audigy A 9800, and the new setting had changed to USB Audio Device, so in the mixer I changed them to the same. I started getting sound, but now I've lost in completely, even in other songs. Can anyone suggest what's happened, and how I can hear the audio again? Thanks.