Im a composer and frequently write fugues on scratch paper to practice my counterpoint and ability to compose away from the piano  The problem is not a lack of understanding on my part  I can hum the tune of the Bachs Art of Fugue  Likewise you could probably hum the tune of A British Tar from Pinafore despite it being polyphonous Im saying that it is not necessary to hum every voice concurrently to claim to be able to hum something especially since a fugue only has usually one or two melodies despite it having multiple voices