How can I get past these trees?

Hint 1 of 12:

Wait until you get past the guard first.  You can't do anything until then,
and the following hints might spoil your fun if you read them prematurely.

Hint 2 of 12:

The first thing that you should do is talk to the people in the area to find
out something about them.

Hint 3 of 12:

Someone will tell you that the trees used to be part of a musical group.

Hint 4 of 12:

You're going to have to find some way to make the trees remember their past.

Hint 5 of 12:

A musical instrument might help.

Hint 6 of 12:

Have you done a good deed yet?