Hello Arnold!
This problem may have different reasons. Please check the following things.
- Did you reserve a program/channel for all instruments of your score on the Sibelius/Manual Sound Sets page?
- If you have an older version than Sibelius 6.2, please update to Sibelius 6.2. Some playback issues have been resolved in this version.
- If you have made channel or output settings in the Sibelius mixer, please reset all outputs to (Auto). Making manual settings there can mess up your whole playback.
More help can be found in the Troubleshooting section of the "Optimzing Sibelius Playback" manual, which can be downloaded here:
http://www.vsl.co.at/en/68/141/460/576/655.htm
Best regards,
Andi