Questions about Sonos HDMI ARC to Optical

Unfortunately not, the adapter only works in one direction (Optical -> HDMI). You probably need the Sonos Beam, which has an HDMI e-Arc connection.

The "HDMI ARC to Optical (Toslink Adapter)" adapter is designed to convert an HDMI ARC signal to a Toslink optical signal, allowing devices without an HDMI ARC socket to be connected to an optical audio output. However, it is primarily intended for Sonos products, and compatibility with other brands such as Marshall is not guaranteed. To connect a Marshall Woburn 3 speaker to a television without an HDMI ARC output but with an S/PDIF output, it would be best to check the specific compatibility of the adapter with the Marshall model or use a universal adapter compatible with non-Sonos devices.

So which adapter do I need?

No, the Sonos HDMI ARC to Optical Adapter is not intended for reverse use.