It certainly can be an issue because the US and EU have different WiFI regulations so I could imagine that you might have trouble either connecting the Arc Ultra to your WiFi network or that the Arc cannot communicate with the rest. 

How did you end up with these units? If possible I would contact the seller and get matching units (either US or EU depending on where you are located). 

Hi ​@Lidor 

Welcome to the Sonos Community!

I can confirm that EU surrounds will not play any audio when connected to a US soundbar.

I recommend you get in touch with our technical support team to find out what your options are.

I hope this helps.

Hi, 

Thanks a lot for the help.

If I will change to the European Sonos Arc Ultra model - ARCG2EU1BLK

Is this supposed to fit properly with my European ERA 300 and the GEN4?

ERA 300 - E30G1EU1BLK
SUB GEN 4 - SUBG4EU1BLK

Hi ​@Lidor 

Yes - if you are yourself able to swap the US Arc Ultra model for a EU model, the EU surrounds will then work.

I Have EU arc ultra and US era 300 - work perfectly 

Hi ​@MarkJnf 

I think that is because the US uses channels that the EU does not use, whereas the EU does not use channels that the US does not use.

I had a US Arc Ultra and EU surrounds that would not play.