Hi Timo,
I use Sibelius 7, not First, but I believe the rewire process is the same in both cases.
Launch Sonar only - First should not be running.
There is one thing that you need to look out for and that's in Preferences/Audio/Playback and Recording - you have to make sure "Always Open All Devices" is unchecked. The other thing is that in Sibelius I had to disable the intro sound on open - it launches with a short fanfare and that causes hell on my system.
Insert rewire device, choose First. Double clicking on the audio or midi track will NOT launch Sibelius, it needs to be done manually. under play configuration, DON'T chose general midi. use the Sibelius soundsets unless you have a vst setup in First (see the attached image).
There are a some issues with tempo syncing (i.e. needing to import tempo tracks to sibelius) but there is help on forums for that.
Let me know if this helps at all.
OH - and check this link too. avid.force.com/pkb/articles/en_US/troubleshooting/en362399
Best,
post edited by swamptooth - 2014/08/04 23:24:05