Have you been able to stream Youtube through the USB? Asking to see if you are able to get any audio through USB, or just the one app you want to use isn't working or their app doesn't support that in some way.
What's the brand / model head unit?
I can stream youtube, soundcloud, and other sites with audio from my 6 year old iPhone to my 10+ year old Sony via USB, same for the 10 year old Pioneer I recently put in my truck. I don't subscribe to any music apps / sites, I guess that is a difference, so I don't know anything about the site you want to stream from, but I have been able to stream through USB. I mainly listen on Youtube.