How would you explain the metaphor of the hypodermic needle in the fifth stanza of "Trumpet Player"? Does Hughes mean that music is the needle that touches the soul of the performer? What is the effect of this technique?

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.

In the last stanza of "Trumpet Player," Hughes writes that "It's hypodermic needle / to his soul." Yes, this means that the music the trumpet player makes soothes some of his pain and acts like a tonic to him. The effect of this metaphor is to emphasize how strongly music acts upon the performer. It's also a reference to the earlier lines that read, "that is longing for the sea / where the sea's a bar-glass / sucker size." In these lines, the trumpet player's desire for the sea, and his desire to overcome his pain, cause him to turn to a "bar-glass," or to drinking. The contrast between the bar glass and the hypodermic needle is that the trumpet player is drowning his sorrows by drinking but the music, like a drug, lifts his soul. The poem is about the ways in which the trumpet player's music is an expression of both joy and pain, particularly pain from the past, including slavery. The metaphors of the needle and the bar glass continue the idea that his music combines elements of both joy and pain.