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Brother Preptil, the master of the music, had described Brutha’s voice as putting him in mind of a disappointed vulture arriving too late at the dead donkey.
— Terry Pratchett · Small Gods

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Brutha's voice is compared to a vulture that has missed its meal. The image makes his singing sound both unpleasant and full of complaint. Such a strange, detailed insult is what gives the description its comic force.
Context reviewed