Most FM stations are going to be available through tune in, iheart radio, or other streaming service. I would look up the website for the station you want to hear to find out how to access them.

The amp does have an aux input, but it is not active if your amp is bonded to the soundbar for surround sound duty. So if the amp is it's own separate room, and your receiver has output,  then you can play content from your receiver on Sonos. But that is not the best way to hear your radio stations

So when the sound bar is off Yamaha on aux input will work?

The soundbar being on or off,  assuming you mean playing audio or not, is not relevant.  If your  amp is bonded to the soundbar to play surround with the soundbar, you cannot use the aux input.  if your amp is setup as it’s own Sonos room, completely separate than the soundbar, then you can use the aux input.

Reading this may clear up some confusion.