Explain why the offspring of a cactus finch and a medium ground finch would look like an intermediate between the two birds, but the song would not be an intermediate.

Yes, the offspring would be an intermediate in terms of phenotypic characteristics. An intermediate in terms of the varied finch size and shape of the beak (defined by with variations in their DNA sequences). However, the song would not be an intermediate as it's song is unique to Galápagos finch family tree. It's a genotypic character that is manifested in Galápagos finches species at the early part of their life.