i'm 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 bach's art of fugue likewise you could probably hum the tune of a british tar from pinafore despite it being polyphonous i'm 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