There is an unavoidable lag. 

It’s called grouping speakers: https://support.sonos.com/en-us/article/group-and-ungroup-rooms

And yes, the lag is unavoidable.

Thanks. So the only solution to have a synchronized sound without lag, with a Beam on another speaker, is not to buy a Roam but a One for example?

You’d need two One SL’s (or IKEA Symfonisks) and set them up as surrounds if you want extra speakers but no lag. Remember surround speakers are exactly that: they play birds singing, explosions and people applauding, but no conversation if you use them for TV sound. For music they can be set to full, which makes them stereo, while your soundbar will also be stereo.

Thanks. It is difficult to understand why we can have the same sound in 2 different speakers with no lag when listen to music by AirPlay but impossible not having lag when the sound codes from the Beam connected to the TV.

The way Sonos works is to buffer an input for a period of time, in order to be able to transmit that input across a LAN to multiple points of playback in sync.

This paradigm broke down when they developed speakers that had to sync with the video on a TV, so they created a special case where that one ‘room’ used the digital input, and a special low latency 5Ghz channel to communicate between the soundbar and the (potentially) associated surround speakers and the subwoofer.

However, when that signal needed to leave that room, and be extended to any other room, it became subject to the need for the potential synchronization as all other Sonos data, and as such, subject to that same buffering delay, so that all (potential) other rooms can play in sync. This is why any ‘grouped’ room is slightly delayed with a room playing a digital input. 

However, when you’re streaming music, that ‘lipsync’ process is no longer necessary, all of the data stream is already ‘buffered’, and all rooms play in sync.