http://schema.org/MusicAlbum and http://schema.org/MusicRecording use http://schema.org/byArtist to designate the artist credited with an album. Currently, the only acceptable type is http://schema.org/MusicGroup.

While MusicGroup's description says it can be used for individual artists, it is a subtype of http://schema.org/PerformingGroup, which does not.

Given it makes distinguishing groups from individuals, the simplest thing would be to expand the range of byArtist to accept http://schema.org/Person as well.