post {protocol}://api.ws.sonos.com/control/api/{version}/groups//playback/skipToPreviousTrack
Use the skipToPreviousTrack command in the playback namespace to skip to the previous track.
Not all audio sources allow multiple tracks. For example, when a group is streaming an Internet radio station, there is no previous track to skip to. If you send a skipToPreviousTrack command when the audio source does not support multiple tracks, your app will receive an ERROR_PLAYBACK_FAILED, but audio will continue playing.