"on speakers" is correct and usable in written English.
You could use it to describe the source of a sound, for example: "We could hear the music playing on speakers in the background."
People climbing on speakers.
The last post played on speakers.
Cast in song, their dialogue plays on speakers in this debating chamber.
Some good ideas, via the Telegraph, "Nick Cave hopes to hand out books on Speakers' Corner.
Soft music, heavy on sax and synth, played on speakers that were directed toward the stands.
Helicopters whirred overhead through the service, which was broadcast on speakers to the crowds outside.
Digital tricks impressed a pattern of delays on speakers throughout the ballpark to do just that, he said.
It meant putting staff doctors and the embryology director, Kathy Miller, on speakers' boards for companies that make fertility drugs.
He also worked at an electronics store, where he gave up-and-coming musicians discounts on speakers.
By the opening of Magdalena, he's climbing on speakers, pointing, bopping and doling out those contagious Milky Bar Kid smiles.
You can hear him thinking (his voice plays on speakers) as he describes designing a handbag: "I'm in a position of being respectful and disrespectful," he says.
