Maybe with an "USB AC charging hub", that has a 2.1A 5V output on it, but still, I'm not sure if USB power-using devices still have to somehow "identify" with the USB port using some kind of logic, before the power gets allocated and starts to flow to that port? Might run into an issue with that. Then again, they do make USB-powered speakers (that still use a 3.5mm phonojack for sound input), and I don't know if they have any sort of USB logic chip, given their low cost, I doubt it.
Is it that hard to find a replacement wall-wart (brick) on ebay for those speakers? Look for router power adapters too, those might cover the one that you need. Shouldn't be more than $10, $5-6 from a China vendor (whom I DO NOT recommend.)