Philadelphia’s Kimmel Center for the Performing Arts has been open for less than four years, but an internal report by the acoustic engineer of Verizon Hall, the center’s main stage and the home of the Philadelphia Orchestra, suggests that a major interior renovation will likely be necessary to fix what are described as “serious acoustical problems.” The hall has received mixed reviews from critics since its opening, but the concept of a major renovation is likely to face opposition, and debate has not even begun about who would pay for such a project.