Langston Hughes's "Trumpet Player" is about a musician who uses his instrument to express and soothe the passions and desires of his soul. Music for him is at once a creative form of self-expression—through which he expresses his individuality and his heritage—and a form of healing, which allows him to express rather than repress feelings which need an outlet.
The metaphor of the hypodermic needle begins in the fourth stanza and continues into the fifth stanza. It begins, "The Negro . . . does not know / upon what riff the music slips," and continues, "It's hypodermic needle / to his soul."
A hypodermic needle is used either to inject substances into the body or to extract substances from it. Both uses are relevant to the metaphor in the poem. The music that the trumpet player plays at once feeds (or nourishes) his soul with its "honey / mixed with liquid fire" and draws upon (or from) the passions and longings of his soul.
It is very much a two-way process; it reflects how the soul of the trumpet player and the music that he plays are, essentially, one and the same thing. They are co-dependent and necessarily inextricable. One is an extension of the other, and both are fundamental aspects of his identity.