According to this poem, when strangers are near, pleasure "sings and laughs." When it is alone at home, though, pleasure is "cold and dumb." This goes with the main idea of the rest of this poem.
The major theme in this poem is that joy is superior to pleasure. Joy is something that really comes from the heart. It is a pure and healthy emotion and one that can sustain itself. By contrast, pleasure is unhealthy. It is more hedonistic and does not really have a spiritual side to it. It is something that is superficial. The fact that pleasure is this superficial thing is what makes it sing around strangers but be dumb when it is by itself. It is something that depends on others rather than something that is experienced by people who are more mature and self-sufficient.