No sound for participants when sharing audio from I-Phone

I would like to share sound of a language app or I-Tunes on my I-Phone. My students can not hear the sound from the I-Phone, but can hear the sound when playing a video that is shared from the Internet. 

I click on share screen and always enable share sound before I connect with Airplay, just as described in the tutorial.

It worked in the past, but since the last four weeks or so it does work anymore.

Thanks for your suggestions.