Your Arc Home Theatre system will likely have a room name, like ‘Lounge’ for example, as the devices are ‘bonded’ together as one single room.

Any other speakers you add to the Household will usually have different room names. You can ‘pair’ like-speakers to make them a stereo pair with their own unique room name too, or you can just have a speaker stand-alone.

In the Sonos App you can ‘group’ any or all Sonos rooms together to play music audio in perfect sync around the home, or play different audio to different rooms.

TV audio can be played to grouped rooms too, but it might have a slight delay when playing to other rooms but normally if the audio is playing in say a separate room next door, you will unlikely notice the slight delay.

So I have to un-group and group each time I want to switch between surround and music?

How come it doesn’t automatically understands what input I'm using? I mean, if I play a movie it should go into surround mode and “cut off” the speakers that is not using the benefit of surround, and if I switch to music it should use all the speakers.

You can set to TV Autoplay, so when the Arc sees the tv signal start it will switch automatically. You can also set the system to Ungroup on Autoplay if you want it configured that way. 

You just can't set it to group automatically when not playing TV sound. Grouping always needs to be done with a tap in the app or on the speaker you want to group.

That’s too bad, I kinda hoped the system would be smart enough to do that for me.

Unfortunately it is incapable of mind reading, and some people may want to have TV audio playing in other rooms sometimes and sometimes not, or group different rooms at different times.  Yes it could include more options or defaults, but every option adds to complexity.  I think it’s smart enough, you are of course entitled to think differently.  Perhaps when you have tried it you won’t think it’s a big issue.

With this attitude nothing would progress forward. 

It’s not that complex. Say I have 10 speakers, 4 of them are connected to a group called Cinema, and the rest spread out in different places around the house each in their own group. Say I merge them in one group called Home, and from there it understands when I’m using either Cinema or Music. 

When playing a movie, it plays the surround sound from the speakers that are included in Cinema, and when playing music, it plays music from all the speakers that are included in Home.

Much like how I use LifX lightning in my house, I either use them individually in their own group, or all together as a Home group without having to manually switch between groups.

If you choose to use voice control and use Amazon Alexa ‘enabled’ Groups’ you can actually make the Sonos ‘rooms’ group/ungroup automatically depending on the Alexa device you speak to and the instructions given… or you can use the features in the Sonos App to do those things. There’s a bit of a learning curve to begin with, as there is with any smart-home product, but Sonos with Alexa works very well I think.

There’s is also group volume control or individual room volume control using voice or the App.

However don’t mix up ‘grouping’ with the terms ‘pairing’ or ‘bonding’ as each of those are different things... you can stereo ‘pair’ speakers and ‘bond’ speakers together in a Home Theatre setup.

So you can ‘group’ standalone speakers, for example with products that have already been ‘paired’ or ‘bonded’ together… the ‘grouping’ is quite fluid, whereas the ‘pairing’ & ‘bonding’ is a bit more of a (semi)-permanent type of linking. 
 

As an example, just to perhaps help explain, you could have a stereo pair of speakers in the Kitchen, a bonded HT setup in Lounge (Arc, sub and 2 surrounds) and two stand alone speakers in the Dining Room and simply say to Alexa “Play some music downstairs” and if the Alexa groups are setup correctly, every speaker mentioned (part if the downstairs group) can be made to auto-group and play the requested music.

You could the goto the Lounge ..and either using the TV remote or another Alexa voice instruction, switch on the TV and the TV will automatically break apart the earlier (downstairs) group and play to the connected Arc, sub and two surrounds only in that one room .. there are however other settings to make them not break out out of any previous group - so there are some customisation options to fit most users needs.

I hope that additional information helps.

I use Google Assistant, I'm guessing it works just as good after a bit of tweaking?

Yeah, that was exactly the solution/function what I was looking for. 
Thank you!