A harmonica was a musical instrument.
During a World War II bombing raid in London, Will Hoffman played a tune from The Muppet Show while sheltering in the White Rabbit cellar. Emily Chaudhry was angry with him for potentially changing the past with an anachronistic song, but the Sixth Doctor said the song was probably older than The Muppet Show. (PROSE: Incongruous Details)
On Terra Alpha, Earl Sigma played the blues on his harmonica. (TV: The Happiness Patrol)
In the Yssgaroth's universe, the Seventh Doctor played "Jimmy Crack Corn" on his harmonica. (PROSE: The Pit)