I am writing a marching band piece where there is actually a place calling for a scream (either the band does it or a recording). Is there a way I can input a scream into my score so it will play back, when I make an mp3 of it? I hope this question makes sense. Yes, the movement is called ";Scream";.
Here's what I would do: Go to the internet and download a sound effect of the sort of scream you are looking for. Many are available for free and others cost a dollar or two, but you should be able to find exactly what you are looking for.
Create the wav file as normal in Sibelius (only don't put any audible ";cue"; where you want your scream). Open it up in Audacity. Then also open up your scream sound effect. Splice and dice until you get it where you want it and get the balance and pan just right. Then mix it down.