This sounds likely. You’re right that they are hidden classes, but they’re being returned by non-hidden methods - so it’s all a bit strange. I’m pretty much following the logic flow presented in the official tutorial : https://docs.oracle.com/javase/tutorial/sound/sampled-overview.html
You get a list of mixers-infos from the audio subsystem, choose one and get it’s corresponding mixer. Then you get the mixer’s line-infos, choose one and get it’s corresponding line. Finally you open the line and get the data.
But the weird part is that I can’t get the line b/c the mixer I’m getting is this hidden class type
I’ll keep digging around and if I discover anything new I’ll drop a note here 
Thanks for the help and pointers. Really appreciate it