Are the 300s bonded to the Arc as surround speakers?  Group is a different term that implies you chose the Arc as one Sonos room and the 300s as the second Sonos room in the group.

If they are Bonded into a single Home Theater Room I’d wait for the drops to happen and submit a diagnostic. Note the number provided and call Sonos Support to have them look at your problem.

Are you sure the YouTube sound is surround and there is actual sound on the rear channels? Not all are and that could be masking the issue.

What you are experiencing with movies is normal behavior when Grouping other Sonos speakers to a Sonos soundbar. There is a 75ms delay which seems to further manifest itself as drops in your situation. On the other hand not experiencing the same when playing music is also normal behavior. 

Hi MoPac! Thanks For the reply!

Do you mean that i have to go ARC settings -->Set up Sourrounds and make the bonding?

I did it and it says to me “Lets add sourrounds to TV Room Arc” → Continue → and it says to me “Two compatible sonos speakers are required to add surrounds. Make sure your products are powered on and connected to Wifi” . And it doesent let me continue.

To let you know, i have already pair the two Era 300 (L/R) and i dont have a sub. 

Is the sub gonna solve me the problem(?) , if i buy one?

Thanks a lot!

Hi again!

I solve the problem guys! 

I Unpair the two ERAs 300 as a stereo system, and i pair them seperately as a surround system in my TV Room with the ARC soundbar!

Thanks a lot for helping me!!!

If you’ve stereo-paired the eras you need to separate them. Then they should be available. 

No, a Sub will not “solve the problem”. Just set up the home theatre system following the guides, or on the advice you’re getting above.