Hi, I am A. Stevens here to help with your question!
Most likely what has happened is an internal component on the keyboard has failed, therefore why you are experiencing the sound quality issue with both the internet speakers, and external sound as well.
How recently did you purchase the Roland Piano?
It would be most ideal to get Roland or the outlet where you purchased it to repair/replace the Piano.
Please let me know when you return, so that we can continue working on this question. I will respond as soon as possible when I see your reply.
Thank you. I was hoping you would be able to suggest some components to check on this model on the basis of which are the most likely ones? Have you got access to the circuit diagrams for this piano? Does it have separate output transistors? Do the symptoms suggest they are probably ok as the other outputs - to headphone/external amp & speaker- are also low? If so, what likely component would that point to?
Let me try to locate that information for you.