According to the Sibelius 7 manual:
Troubleshooting
If Sibelius does not appear as a ReWire device in your DAW, choose Play Setup Playback
Devices in Sibelius and click Audio Engine Options. Click the Repair button to fix the ReWire
association, then quit and restart your DAW.
If the ReWire connection appears to be working correctly (e.g. both applications start and stop
playback correctly) but you find that you are hearing no audio from Sibelius in your DAW, check
that the current playback configuration in Sibelius is set to use virtual instruments: the host
application will not receive any sound from MIDI devices, whether they are internal (e.g. your
computer’s built-in soundcard) or external (e.g. a sound module).
If you find that Sibelius continues to start up in ReWire mode even after you have quit your DAW,
choose Play Playback Devices and click Audio Engine Options, then click Clean Up. Now
restart Sibelius, and it should start up and operate normally once more.
Using the 64-bit version of Sibelius with ReWire
ReWire is only available in 32-bit applications, because the underlying technology by Propellerhead
Software has not yet been updated to 64-bit. However, because Sibelius uses an external component
to manage its connection to the ReWire host, you can in fact run the 64-bit version of Sibelius and
still use ReWire.
However, the ReWire host application must still be running as a 32-bit application in order to make a ReWire connection with Sibelius or another other synth application.