Layman explanation:
The valves in the heart keep the blooding flowing in the proper direction. When a valve doesn’t function well some blood leaks “backwards”. The mitral valve is a specific valve in the heart. My guess is your thickening is causing a murmur—back flow. I’m not absolutely sure it would cause a murmur or prolapse, I just know some Mitral Valve Prolapse (MVP) is caused by thickening.
People with MVP often are recommended to be pretreated with antibiotics before dental cleanings. If your doctor has other concerns about it I’m sure they will tell you.