That looks like asphalt/tar coated pipe.
The sinkhole could be a tree stump that is decomposing.
First thing I'd do is call around to different plumbers or sewer guys and see about running a camera thru the pipe again, this time better resolution. No need to do anything until you know it's bad. Why spend the money to fix something if it ain't broke?
If it is bad, look at getting the pipe filled on the pond side with hydraulic grout and installing a siphon system.
Next get to work on the cattails and lilies.
