In a recent blog post I tried to define a Work as:
A Work is a musical composition that will at some point be performed and possibly recorded, in which case it will become a Recording.
This definition makes it difficult to address compilations, DJ-mixes and mash-ups. What if we're more liberal in our definition of Work:
A Work is a new musical creation or a creative combination of existing musical creations. Works can be performed and recorded to create recordings or recordings can be creatively combined to create new recordings.
By allowing a wider definition of a Work we can capture more information about the Work. For instance, if a compilation of music is considered a Work, then the release level information/ARs can capture the information about the recordings contained in the compilation and the Work level information/ARs can capture the information about the compilation itself.