I've had this problem consistently for a while now on Sibelius 7, but I can't handle it any longer! Wondering if anyone else has had this same behavior.
Whenever writing timpani parts in Sibelius 7 (using the 6.0a template) Regular notes, rolls, etc. play back perfectly fine. When I add an accent, however... the playback switches to the Sibelius 7 Sounds timpani sound... but ONLY for accents. I've tried looking in Edit Instruments like I would to change something in a non-pitched staff, but since it's pitched it doesn't have any assignable sounds per articulation.
Only thing I can find that I think might affect it is in the playback dictionary (turning off the Sound ID identifier for accent) but I don't want it to be global and mess with the built in Sibelius Sounds brass library.