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When playing music on TV channels the rears are still surround speakers (so play very little info normally on a stereo music channel).
When playing music from the Sonos app or online service the rears (when set to full mode in room settings) will be full stereo sound and will sound terrific. But not from music played from TV (as there is no way to activate full mode when playing via HDMI input to Beam - always in surround mode).