Yes they're plastic. The LP's metal part is only on the exterior, it's still screwed into a plastic piece.
On a random unrelated note, I purchased a Retina Display MacBook Pro and its "improved" speakers are trash compared to my older MacBook and my 6-year old Dell Vostro. Tinny, sizzley treble, no bass, no lower midrange...just terrible.
The M-100's are the perfect companion to these "speakers" as you get the bass you miss from them. XD