Check the manual for your Yamaha. Some devices have line level outputs for zone 2, some have amped outputs. If it is indeed line level (which there is a good chance), then you can connect it to a Sonos line in without issue.

The Sonos controller app will control the volume of your Sonos components in that case, and not the volume on your Yamaha.

Also, it’s not uncommon for this second zones on AVRs to be analog inputs only, or something similar.  Meaning that you can’t send audio from an HDMI or digital input source to zone 2.  Not sure if that’s the case for you.

Also also, you can have the Connect/Port be an input for your receiver as well as output.

Thank you Danny and Bruce. I have delved into my Yamaha settings and the Zone 2 output can be set to fixed volume. 
Am I right in thinking this is like an old Tape Out and will be fine if I connect it to my SONOS analogue input? There is a Pre Out as well but don’t know if that is fixed volume.

Yes, tape out should always be at ‘line level’, just as this output should be. It is normally designed to feed another amplifier, which is supposed to be looking for that line level signal. 

In a normal receiver, all outputs except the speaker posts should be line level. All inputs should as well, except for the one that is labeled ‘phono’, which goes to a built in pre-amp to boost the signal to line level so it can be dealt with the same as all other inputs. It’s the last hold-over of some ancient (pre-standardization) tech.