Hey, Brook's music has the power to make people see and hear things, remember? Obviously his music was bewitching his audience into hearing a full band accompaniment. Me, the part that stood out to me about that sequence was Luffy having a "what the fuck?" look on his face the whole time.
The first third of this episode was truly rather painful. You don't get more "requested by the committee who don't actually work on the show" than that. Thankfully once that obvious padding with the bee swarm was done things picked up a lot and more character and cleverness began to show through in the directing and storyboarding. Just please…all the good directors on this anime need to file restraining orders against Deguchi. So much sloppy stuff in the first half, like everyone's literally long faces and Carrot having Rei Ayanami's haircut.