For some reason I cannot get the volume right in GOP. Even when the volume mixer shows the slider almost on zero, the playback is extremely loud and sounds distrurbed. Also I hear to much echo, no mather what I change in the Ambience settings. Has anybody had these problems?
Andre, I use the same system (Sibelius 3 & GPO). I sympathize. I am still fighting with the same issue. The organization of it is a bloody mess. It's only logical if you have been doing digital audio for years. Volume is controlled in several different places and there is no central source of documentation for this. You are compelled to figure it out yourself.
Your sound card volume control is kind of a master controller. I raise mine as high as it will go.
The mod wheel on the Kontakt keyboard is one volume controller. But its control varies depending on whether you have set GPO Studio mode to Default or Ignore Mod Wheel.
The sliders on voices in the Sibelius mixer are another, but as you have observed, they sometimes don't work with Sibelius playback. I have not figured out when and why. They work fine when I play back with the voices in my soundcard, so for this and other reasons I usually use the soundcard for development till I'm ready to make a recordable version.
The volume control that may be your biggest problem is the circular Volume dial on the Kontakt player. You have to adjust it individually for each patch. It is a ridiculous design, hard to turn with a mouse. Often you have to move the mouse up to move the dial down, or vice-versa.
Dynamic markings f, p, mf etc do not work with Sibelius though I think they do in the new Finale, which is a very attractive feature that may by itself make switching to Finale worthwhile. (Though that means another learning curve, and who knows how good the integration with GPO and the documentation are.) There is, however, the option of embedding MIDI volume commands in your score, which I am still figuring out. See the Sibelius 3 manual, page 338. I put ~C1 (volume) commands right next to my p, mf, f notation text with the hidden command feature, using a tilde; see page 339.
Trumpet82 recently posted a useful chart of controller commands (including volume), usable in Sibelius, at: