It could be meant literally.
You know the way that sirens go Waaaaoooooo....Waaaaooooooo... or something like that.
Imagine lots of them all doing that but not all in time with one another. They are out of phase.
Now imagind that instead of not being in time, they all Wa. in time with one another - in phase.
If they are all in phase then they all start the Wa.. at exactly the same time.
I suspect the meaning is more figurative, but this literal brain can't think of a figurative meaning just now.
Perhaps this refers to the Doppler effect? If the cars are racing towards you, the sound is, to speak imprecisely, compressed. They sound "faster." Once the cars are slowing down and stopping, the sirens all run at the same pace--though not in unison.