I imagine you are referring to this question:
¿___ debemos escuchar? (Whom do we have to listen to?)
When you listen to something, for example, "Escucho la radio todos los días" (I listen to the radio everyday) you don't need "a", but when you listen to someone/people, it is "escuchar a...". In this case we are referring to people, in the form of a question, therefore you need "a", ¿A quiénes debemos escuchar?
I hope this clarified your doubt.
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