You need to do some diagnostic tests.
Disconnect all the Pis from the hub.
Is there still noise in the speakers?
Power one Pi from the hub, do you get the noise?
Power one Pi from a phone charger or other 5V USB source, do you get the noise?
Disconnect everything except power from a pair of speakers, do you get the noise?
You need to try one thing at a time to narrow down the source of the noise.
I have a pair of Audioengine powered speakers, used them for years at work, used them for years at a home, no issues, no noise.
Moved to a new home in a rural town, noise/hum/buzz all the time, even when in sleep mode. Moved the speakers to a different room and power circuit, no noise.
Moved again to a new home in an inner city suburb, no noise.
It might just be the power to the speakers.