Memphis, oddly enough, doesn't have an "official song," but it should given that it is, supposedly, mentioned in more songs than any other city in the world. Kind of a tough choice... you have the obvious selections (no need for Elvis or Justin Timberlake)...
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Depends on who you ask though... a country fan might say one of these:
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Meanwhile, a big fan of old school Memphis style rap might say:
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...while a younger hip hop fan would almost certainly go with this... "901, Shelby Drive look alive look alive..."
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... but really, you could make an argument for so many other songs too.