Cerium-based type-I clathrates exhibit unusual thermoelectric properties, making them promising candidates for energy applications. The origin of the thermopower enhancement observed at elevated temperatures in CeBa7Au6Si40 remains a subject of active debate, particularly given the coexistence of Kondo scattering and prevailing Curie-Weiss magnetic behavior down to approximately 1 K. To elucidate the electronic mechanisms contributing to these properties, we compared optical conductivity (sigma(w)) measurements on CeBa7Au6Si40 and its rare-earth-free reference, LaBa7Au6Si40. Our ab-initio band structure calculations successfully explain sigma(w) for LaBa7Au6Si40. In stark contrast, the optical conductivity of CeBa7Au6Si40 undergoes a significant change near T=100 K, a feature that cannot be explained by conventional band theory, indicating strong 4f-electron correlation effects. These findings underscore the necessity of a theoretical framework that captures the correlated interplay between 4f-electron physics and the rattling modes of guest atoms. We propose such an effective model and calculate sigma(w,T), offering a comprehensive understanding of the electronic behavior in these complex materials.