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