I'd think artists are commissioned, as I've never read anything about there being an official Disney Orchestra. The only video I've seen of music actually being performed was the video of World Of Color being recorded at Abbey Road.
Well, Since i found out that they have or create a orchestra to recreate music for rides such as The little mermaid, i really want to be apart if not i should make a orchestra called Disney fan music lol
Here come's the bride. As long as we live. Till death do us part. You may kiss the bride.