Sibelius presently only permits two-channel audio playback (that is, stereo). This significantly limits high-end Sibelius users--composers, arrangers, producers, and performers targeting surround-sound (e.g. 5.1 or 7.1) audio systems, or larger speaker arrays.
This is particularly significant with very large VST libraries where different portions of a library (e.g. strings, winds--or different ranks or divisions of an organ) can be assigned to a different array of speakers. Many DAWs can support multi-channel playback--Sibelius should, too.