SSP CP33 Sibelius 5 User Manual page 21

The explanation lies in the way Sibelius allocates sounds. Sibelius determines how to
allocate a staff based on the sound IDs available in the sound set, the Best Sound de-
fined in the instrument staff's settings, and any manual sound set or Preferred Sounds
directives present in the playback configuration. This is repeated for each staff in the
score until all staves are allocated to an appropriate device and sound.
Manually selecting a sound or device in the mixer tells Sibelius "Use this sound regard-
less of any other setting." This overrides the entire allocation system, and while Sibelius
will still work within the system, it's forced to ignore certain things in order to comply
with the manual assignment made in the mixer.
The trouble caused by doing this is not often obvious, in fact, depending on the
complexity of the library or device, problems may not appear right away. Nevertheless,
since there are ways of controlling and guiding sound allocation within the system, in-
cluding manual sound sets, Preferred Sounds, and instrument staves, it's best to utilize
the methods provided rather than "brute-forcing" and overriding the system through
the mixer.
Yamaha CP33 - Sibelius Sound Set User Manual | 21