I wrote a song 9 years ago and I noticed that it is very similar to the song in the Disney Parks commercial that started airing last year. I had the song on myspace.com and shared it for free with many people. The end of the song sounds like the portion used in the commercial. I was wondering if I had any grounds to take some sort of legal action. I recorded the song at a studio and I have a copy that unopened that I mailed to myself (dated).

Copyright infringement does require "copying," which means that you would have to establish that Disney had access to your song and made a substantial copy. A second hurdle is that you must register the copyright in your song before you can enforce your rights. Several important rights are only available if you registered the copyright in your song BEFORE the infringement began, including the right to ask for attorney's fees if you should win an infringement lawsuit.

There are many factual considerations in deciding whether there is copyright infringement or not.  The test is whether someone had access to your work and whether there is a substantial similarity.  The only way to tell is to consult with a copyright attorney.