I'm afraid 'brick is pulling your leg on this one. I've been listening to and watching baseball for about 50 years and I've never heard anything sung at the end of the game. This is a reference to an opera not being over until the last aria is sung, usually by the fat lady; it is an unfortunate truism that the diva is usually... zaftig -- that's how they generate those booming, dulcet tones.

Once again, Quinion has the real story on this one, including the relation to Yogi Berra's "It ain't over till it's over."

http://www.quinion.com/words/qa/qa-ita1.htm