Rocket Player no longer supports playing audio when headphones are plugged in on Oreo. :-(
Rocket Player relied on a tiny headset service that listened for the headphone plug in event. In Oreo, we'd have to have a permanent notification in your notification bar just to detect when headphones are plugged in. I filed an issue with Google so that they could simply wake our app up when headphones are plugged in.
https://issuetracker.google.com/issues/65672608Google declines to fix this. Feel free to star this issue and in a nice way let Google know you want this feature back.