A study question I need to answer: "Epic and expressionist features in Death of a Salesman; function of music?"I don't know if "epic" refers to epic as in Epic Theater (Brecht) or as in epic=prose.
I am going to attempt to clarify the Brechtian epic theatre approach to your question. Since you are unsure of the question's intent, please feel free to ask a second regarding the epic prose intent.
Music plays a crucial role in grounding Miller's gestus. Willy's memories of his father's flute adds a psychological/subconscious layer to the protagonist's behaviour. (Essential Passage 1: Act 1 http://www.enotes.com/death-of-a-salesman/essential-passages-by-character-willy-loman )
Music alters Willy's reality. Audience members are aware that music is a precursor to Willy's flashbacks. The boundaries of the Loman Brooklyn home have no meaning once the flute tune is heard. Another Brechtian epic feature is fragmentation which the flute music creates in a realistic manner. Music transports us without warning. Willy's "ticket to ride" into the past is a tune created by his absentee father.
The whistling (Biff/Willy in elevators, etc.) could also be commented upon to garner support for music's contribution to Brechtian's epic features in Death of a Salesman.
Good luck, I hope this gives you a start.