I installed the Basic Steinway yesterday & it worked wonderfully playing my compositions in Sibelius.
Today I read the manual :-) and started tweaking knobs.
Consequently I have no sound during Sibelius playback. I do have Steinway sound when I check under Sibelius' 'Play, Playback, Test' routine.
What I do see is that in the Steinway Mixer window, at the top left, there now is a 'slot' that says, "empty slot0" and drops down to reveal all numbers to "empty slot127". Yesterday that slot was greyed-out and inaccessible.
How do I remedy this and what did I do to cause it?
There's two things you should have in the Steinway interface to get sound
1) You should have a microphone position loaded on the Main tab. For Basic edition, you just load "basic".
2) The Ambience reverb "Dry" slider should be raised at least partway (default is about 3/4 raised).
If the Steinway worked previously for you, those are the only things related to the user interface that would stop it from working.
I have the feeling though that you didn't mess anything up in the Steinway's user interface. My guess is that you either changed something in Sibelius that then prevented you from getting sound, or that even you previously heard a built-in piano sound (rather than the Steinway) from Sibelius when it seemed to work earlier. I'd say probably that you'll want to check the Sibelius' virtual instrument set up, particularly with respect to the Steinway plugin. The Sibelius manual probably has good description of what to do to load a virtual instrument.
And though I suspect it's a Sibelius setup issue, please do keep in mind item (1) above.