Vaughan Williams' music has been termed 'nationalistic', or 'spiritual'. Would you construe these terms as metaphorical? They have been used to decribe and categorize his music, have been seen as attributive, and his music has been known for these qualities for generations. I would really appreciate a comment on your view of 'nationalism' as metaphor for a body of music.
I won't comment on Vaughan Williams's music in particular, but I certainly think music that evokes and celebrates a certain nationality can accurately be described as 'nationalistic'. And I don't see that such a description is any more metaphorical than many of the other descriptions we employ with music: 'sad', 'anguished', 'triumphant', 'relaxing' and also 'spiritual'--though I won't say anything more about what might make music spiritual. One way--though perhaps not the only way--for music to be nationalistic would be for it centrally to include certain melodic, rhythmic, harmonic or intrumental elements that are unique to a given nation. If a composer includes such elements because they will be recognized as deriving from that region, and she moreover uses them in an approving or celebratory fashion, then I think her music is nationalistic. And I don't see that describing it this way is any more metaphorical than rooting for that nation's soccer team, or pridefully ordering its beer.