Just use a jigsaw and patiently follow the line. Ragged edges, etc. won't show when the speaker is in, but pride dictates doing it nicely. Use the appropriate blade.

There are all sorts of special router accessories that make nice holes. I would use one of those for holes that show, but not just for one hidden one.

Speakers come with the hole size and often with a template. You may have to get a compass (the circle marking kind). Or you can rig up a stick of wood with a nail through it for the pivot point and a pencil at the other end.