The mother of all musical notation programs is still the most powerful and it's extensible via plug-ins to add even more functionality. There's nothing you can't do musical notation-wise with MakeMusic's Finale. The learning curve can be steep, and it's not always the most intuitive program, but there's a sea of people using it and help is only a question away. The output is nothing short of fantastic and it's suitable for anything from straight sheet music to full-blown orchestra scores. Sounds for playback come courtesy of the Garritan Personal Orchestra sample library.
Note: This link takes you to the vendor's site, where you must register to download the demo version of the software. The demo which doesn't save, and prints only one watermarked page. Sounds are limited as well.
--Jon L. Jacobi