Airplay only from iTunes not systemwide

I tried to figure out if something is wrong with my wifi dingle because i couldn’t get airplay to work unless I realized Volumio as an Airplay receiver is only shown as an output option from iTunes/Music app.
I can’t acces it systemwide. Like I can’t choose to output all sounds from
my phone via airplay as it works eith an airport express or apple tv. I’d like to use the soundcloud app for instance and stream the music output to my raspberry b+ … Is there a way to make Volumio accesible as a systemwide airplay speaker and not inly from iTunes?