The sheet music, instrumental music and accompaniments you will find on this site were all made with Finale by Makemusic.
The reason I chose Finale it over its main competitor, Sibelius, is the quality of the playback. Not only does Finale come with a huge selection of excellent instrument sound samples, you can further extend the sounds at your disposal with the Garritan sound libraries, made by the same company. There are other "Virtual Studio Technology" sound samples out there, but in my opinion, none of them comes close to the quality of the Garritan sounds.
In addition to the included Finale sound samples, I use the Garritan Personal Orchestra. I originally purchased version 4, and have since upgraded to version 5. Besides the orchestra sounds, I upgraded the piano sounds to the Garritan Yamaha CLX concert grand.
Having arranged the music with Finale, I can create sheet music in PDF format, and also export a sound file with the instrumental sounds. Finale automatically interprets dynamics, tempo variations and other expressive markings.
I use Audacity to add a voice recording to the accompaniment and produce the final MP3 file.