Wikipedia articles are licensed under the GNU Free Documentation License (GFDL). The general idea is anyone can take a Wikipedia article and build on it providing the new work is licensed in the same way. The new work is called a derived work.
Copyright law has special cases for music and performances. Our understanding of the situation is that when you take a Wikipedia article and tweak the words and add chords, that is a composition. That composition is a derived work of the original article so must be licensed under the GFDL. The recording of you singing it is a performance, which is different. You own the copyright to your performance, but you technically have to pay royalties to the copyright owners of the composition. This is where we’re a bit stuck. We’d encourage you to license your performance under the Creative Commons Attribution license and kind of hope for the best :)
We’re not lawyers so you submit your performances at your own risk :) If you are a lawyer, we’d like to hear your thoughts on the subject. Please contact us.