In the flashback scene when The Spirit Of Jazz gives Howard the contract for "Musical Genius", The contract flies into his right hand and he's holding a trumpet in his left. Yet in the next shot he's holding the contract in his left hand and the trumpet has vanished.
Howard supposedly cannot play an instrument without being possessed by the Spirit of Jazz. However, we frequently see him playing instruments with no hint of being possessed - in the second series, he and Vince are even in a band.