A catheter system and method for treating a treatment site within or adjacent to a vessel wall or a heart valve within a body of a patient includes an energy source, an inflatable balloon, an energy guide, and an acoustic sensor. The inflatable balloon is positionable substantially adjacent to the treatment site. The inflatable balloon has a balloon wall that defines a balloon interior that receives a balloon fluid. The energy guide receives energy from the energy source and guides the energy into the balloon interior. The acoustic sensor is positioned outside the body of the patient. The acoustic sensor senses acoustic sound waves generated in the balloon fluid within the balloon interior. The acoustic sensor generates a sensor signal based at least in part on the sensed acoustic sound waves.