The Functional Requirements for Bibliographic Records (FRBR) proposes a four-tier hierarchy to describe relationships between works and their derivations. Most scholarship on FRBR and musical works has concentrated on classical music within the Western canon, and little attention has been paid to other genres of music. This study takes a bipartite content sample of both classical and non-classical musical works, investigating the issues with both using the FRBR framework. Results indicate there is a blurring of the roles within popular music that can call into question the boundaries of "work" and "expression." The examination of the pieces within the classical canon revealed many FRBR-specific relationships, but also the existence of relationships with no FRBR equivalent, and the examination of non-classical works revealed even more relationships that were either ambiguous or non-existent in the FRBR framework. The study concludes that there may be significant problems trying to tackle non-classical musical works and their mutations with a strict hierarchical model such as FRBR.