Same here with any Sonos Airplay supporting speaker. Works without problems from iphone but not from iTunes on windows.

Seems to be a bug… not sure if it´s one of Sonos or of iTunes.

Hi @dkrinker 

Welcome to the Sonos Community!

Sorry to hear of this issue you are having with playing AirPlay from iTunes to your Era 300.

I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.

It may help, however, to reboot all the devices involved - your PC, your speaker, and your router. Please try this first.

I hope this helps.

@dkrinker 

I did some further investigating and also reached out to German Support. They know about the problem and were able to reproduce it. 
In fact it’s not a general Airplay issue as just Win iTunes is affected. 
Also not all Sonos devices are affected. For me I can confirm that both Eras (100 and 300) and also Move2 have this issue. Other devices like my Beam2, Roam1, One and P5 Gen2 are working via AirPlay via Win iTunes. 
So I think waiting for a fix is the best option.