There is almost no overlap between Scorch and Sibelius for sound output, so your problems with both are either (a) independent, or (b) both down to a Windows XP issue.
If you have not already done so, please restart Windows XP, and try again. If the problem(s) persist, read on:
Then open control panel "Sounds and Audio Devices", and click tab "Audio". In the area "MIDI music playback", check that the "Default device" shown actually exists and is working - change the selection if necessary. If you select "Microsoft GS Wavetable SW Synth", then click the "Volume" button and check that the "SW Synth" mixer slider is not muted or set to a low volume. (NB this muting can happen after playing a MIDI file in Windows Media Player).
Check that your chosen MIDI output is working by using any application that sends output to MIDI (apart from Windows Media Player). If necessary, check that your loudspeakers are active and that you can hear other sorts of sound through them.
Then try Scorch. Before trying to play the score, click on the "Choose Playback Device" button. In turn, select each MIDI output device shown, and click "Test". Choose one that works, and click "OK". Now play the score.
As for the Sibelius problem, check in "Task Manager", tab "Processes", that there is only ONE instance of "sibelius.exe" running. (or, to put it another way, quit Sibelius and check that there are NO instances of "sibelius.exe" running). If there is a duplicate "sibelius.exe" present which does not correspond to the one that you are trying to use, then right-click it and select "End process". If you value your music, do NOT do this with the instance of Sibelius that you are in the middle of using.
Sibelius 6.2, PhotoScore Ult 6.1, Dolet 5.3 for Sibelius, Windows XP Pro SP3 & Windows 7, 2GB RAM