Just to be clear, 17 U.S.C. 101, provides in pertinent part:
A “work made for hire” is—
(1) a work prepared by an employee within the scope of his or her employment; or
(2) a work specially ordered or commissioned for use
as a contribution to a collective work, as a part of a motion picture or other audiovisual work
, as a translation, as a supplementary work, as a compilation, as an instructional text, as a test, as answer material for a test, or as an atlas, if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire.
The point I'm about to make is that unless your signature and the signature of the freelancer appears on the same contract
that the work is "a work made for hire," then it's not a work made for hire. Consequently, you may
have a problem with youtube.com, if you submit the independent contractor agreement that is displayed by elance.com, since it appears to require no express signature from either the contracting party or the independent contractor.
That said, if you have a copyright registration from the U.S. copyright office for the musical work, and you have a copy of whatever contract the freelancer has agreed to containing the terminology "work made for hire," then that is all that is necessary (unless someone other than yourself was responsible for the video editing/production, in which case you would need a work made for hire agreement with that person, which would have to be submitted to youtube.com.
Please let me know if I can be of further assistance.