When you have multiple hardwired Sonos units while still having some wireless SONOSNet will try to communicate to random hardwired units causing multicast traffic all over the place. When you have a single hardwired Sonos then it will funnel that traffic to the single hard wired unit preserving the rest of your network performance.

Having a managed switch allows you to bypass this by forcing your core managed switch to be the root bridge on your network.

A managed switch does not have to be a product that makes you nervous to install. Our Araknis switch is plug and play out of the box.  Many others are the same way. You only have to make configuration changes when they are needed. Adjusting STP only takes a few clicks of the mouse. We did our best to make this process very installer friendly.

You can read more about STP here under Common Tech Support Issues