We found three antennas in the Sonos Play:3, all attached to a communications card on the motherboard and held in place with a soft glue.

Through a combination of the three antennas, the Play:3 communicates over SonosNet, a proprietary wireless mesh network that connects the player to a Sonos Bridge or another player (but not your home wireless network).

The length of the antenna traces are listed on the antenna board (10 and 23.2 mm). The antenna design seems to be that of an Inverted F-Antenna, common in small electronics and implemented to reduce antenna size and improve multiple input multiple output (MIMO) systems.