We already knew he was full of hot air — but helium, too?

Mayor Bill de Blasio’s livestreamed appearance Wednesday at a campaign event in Iowa turned into an episode of “Alvin and the Chipmunks” thanks to an embarrassing audio snafu.

The screw-up distorted de Blasio’s voice as he appeared at the Iowa Federation of Labor and Congress of Industrial Organizations’ conference on giant projector screens via a video link from Gracie Mansion.

“Organized labor in Iowa has been fighting a really, really difficult battle but you’ve been doing it in a way that shows so much passion, so much strength,” the termed-out mayor tells the packed room, unwittingly in a high-pitched voice.

The 2020 Democratic longshot was supposed to be making his pitch in person, but a bout of bad weather canceled his Tuesday night flight to Iowa.

Members of the audience can be seen cracking up and glancing at audio technicians in the back of the room as Hizzoner — unaware of the technical problems — continues his usual drone about “working people first.”

The bizarre incident left the event’s emcee, Charlie Wishman, scratching his head.

“OK, so that was a little bit different. [I] apologize to Mayor de Blasio,” said Wishman, the chapter’s secretary-treasurer. “Our video conferencing was working this morning when [AFL-CIO President Richard] Trumka was on, so I’m not entirely sure what happened.”

Additional reporting by Julia Marsh