Depending on how much you trust a quick Ngram search, sang is about 30 times more common than sung as the simple past form. But sung is very much still valid; all three definitions of sing given by TFD list it as an alternative form.
This usage isn't informal or uneducated, as some commenters have suggested; you can find countless attestations in formal contexts.
You can find it in The New York Times:
Alex Turner, from the Arctic Monkeys, found ways to distance himself from the teenage culture he sung about
In The Guardian:
Covering Low’s songs Silver Rider and Monkey on his next album, he sung the latter with Patty Griffin, which created its own story.
In The Economist:
the song they sung most often was nothing patriotic, but a lullaby in the Kumamoto dialect
In The Atlantic:
The piece they sung was that in which occur the lines [...]
In The Washington Post:
And it was that latter sentiment with which Franklin so mournfully sung that day [...] She sung the opening lines [...]
In Financial Times:
Meanwhile, he sung the praises of Amazon, Netflix’s newest competitor
In The Independent:
That is exactly what the UK showed Trump, and Theresa May, as we sung against everything they stand for.
In TIME:
He didn’t make the competition, but he did become a meme, inspiring numerous parodies, including spoofs by Jimmy Fallon—who sung it as Neil Young—[...]
Conclusion: there are no grounds for calling this usage of sung mistaken, nonstandard, or dialectal.
Edit: In case anyone thinks this usage is a recent innovation, Webster's 1828 dictionary also lists both "sung" and "sang" as valid "preterit tense" forms, but "sung" as the only valid "participle passive" form.