Just configure your DHCP server to always hand them the same adress (static lease). Do you use the built in dhcp of your wifi router? Most of them offer an option to enter a mac address for which you want to assign always the same IP.

Sonos really does not like if components get different IPs after you had them powerless for some time while other components stayed active. I had the same issues you describe until I configured static leases for my sonos components and the smartphones we use to control them.