because the five of us, instruments and all, are about to angle over to the south side of the street
The speaker's telling his fellow pedestrians to "watch [their] step" (36) because he is walking along the pavement with five jazz musicians (and their instruments). That's a big crowd.
Of course, the speaker isn't speaking literally here. The poem makes it sound as if the jazz musicians are actually walking beside the speaker, but what's going on, as we know, is that he is listening to them.
But the imagery that the speaker is using here evokes the jazz musicians and himself as one big group, and all "five" of them are going to "angle over" to the south side of the street. Pedestrians need to watch their step because a big jazz band (plus the speaker) is walking over to the "south side."
and then, in its own tightly knit way, turn the corner at Sixth Avenue.
The group of five is turning the corner at "Sixth Avenue," which is a famous avenue in New York. (See "Setting" for more.)
By referring to himself and his group of jazz musicians as "tightly knit," the speaker gives us a sense of just how connected he feels to the musicians he's listening to on his earphones.
We can also see some more rhythms being created through alliteration going on in these lines, with repetition of the T sound in "tightly knit" and "turn."