Iris, snapcast & iphone - no sound

I am struggling to understand why on my iphone 11, I cannot get any sound via snapcast using Iris but I have no problem with the same broadcast on my samsung android phone. I have tried, safari, chrome and firefox on the iphone without having any sound ( the iris interface works fine). Youtube etc have sound on the same phone / browsers. I have gone through all the prefs I can find. Any thoughts on why it is silent would be most welcome.
Thank you