There is no suggested height from the floor in the manual (US manual link), the only positioning suggestion Sonos has is 4 inches (about 10 cm) below the bottom of the TV. This is important due to the need to leave space for the upward firing speakers. As there are no downward firing speakers, there really isn’t a height requirement, as the Arc is designed to either sit on a cabinet, or hang on a wall.

I’d be more concerned about eyeline to the TV set, and then place the Arc depending on that. 

Hi Bruce! Thanks a lot for your time...

Yes, actually I knew about the 10cm placement under the TV, I don't have this problem because the ARC is positioned in the cabinet further forward than the TV, which is positioned closer to the wall.

The point is that the ARC with this cabinet reaches a height slightly higher than my knees... (sitting on the sofa, of course) and I wanted to understand if it was okay or if it would be a good idea to make it reach "at least" the height of my belly (so raise the cabinet a bit).

Regarding the correct observation you made about the line of sight of the TV: in fact this point is perfectly in line with the problem of the ARC placement, since both would be supported by the same cabinet. So, if I position the TV in a way that is very comfortable for me on the cabinet, I find the ARC a little too low in my opinion (it would go under my knees when I'm sitting on the sofa), for this reason I was wondering what could be the minimum distance from the floor to position the ARC. To raise the ARC at least to the height of the "belly" while sitting on the sofa, the TV would also rise a bit... For this reason I was looking for some reference, if possible (for example that the ARC should not go lower than a certain number of centimeters from the ground and similar advice...).

Thanks again to you and to anyone who wants to intervene and help me.

The height to place the Arc is indeed based on the placement of your TV, which depends on your seating position. See https://www.coolblue.nl/en/advice/determine-height-tv.html for an example. Important quote: “To determine the ideal height for your TV, measure the height of the screen. The principle is that you hang the upper third of the screen at eye level to your viewing position.”

For examples of how not to mount your TV see https://www.reddit.com/r/TVTooHigh/s/SPrzC5oYUY

​@Ignazio 

Not sure if I am just misreading this but you said: “...the ARC is positioned in the cabinet...”. 

Ensure your Arc is placed ON furniture, not IN it. The upfiring speakers of the Arc - and the speakers pointing out of each end - need to project sound into the air, not into wood.

No, no  Rhonny , sarà un problema di traduzione. L'ARC è completamente libero su un mobile, per far funzionare bene sia i diffusori alti che quelli laterali. 

Perfetto!