It's time to cut your wood using a jigsaw. A traditional handsaw can do but using it would be tiresome and can give off unclean cuts. If you are using a jigsaw, try to buy a curving blade for a better and cleaner cut for the round holes for the speakers. A traditional way to cut holes for speakers is to use a jigsaw but first you need to drill a hole inside the speakers marking to have a start for the jigsaw cut.
Summary: Get your jigsaw and drill to cut down the wood.