If you’re referring to ‘grouping’ the Beam to the Sonos Arc room, then the (group-room) audio will be stereo only and also delayed by approx. 75ms - so you would ‘possibly’ encounter an echo if able to hear all speakers from your listening position, although if you intend to place it a ‘long’ distance away - it may appear to play in lip-sync with the video on screen due to distance, which also causes a delay (sound travels at approx. one foot per millisecond, or thereabouts). 

I should add there is a feature in the Sonos Arc ‘room’ settings in the App, which you will hopefully see, called ‘TV Dialog Sync’ that will buffer the Arc TV audio output, which may assist to bring group-room audio into sync, but whilst the speakers may output the TV audio in sync, you may (likely) encounter some lip-sync issues with the video on screen - I tend to occasionally use this myself however, usually when playing audio from music Apps on my TV.