OK ... to dig this topic out of its grave, I'll add ...
From 1969-1992:
Lana Wood (the younger sister of Natalie Wood)
Lana Clarkson (the Barbarian Queen actress who, unfortunately, was killed Feb. 3, 2003 by "Wall of Sound" genius Phil Spector)
The original Brady Bunch child cast of Maureen McCormick (Marcia), Eve Plumb (Jan), Susan Olsen (Cindy), Barry Williams (Greg), Christopher Knight (Peter) and Mike Lookinland (Bobby) - only the eldest of the Brady sisters and brothers were accomplished singers.
For the 1992-present period:
Denise Crosby (Lt. Tasha Yar on Star Trek: The Next Generation)
Brent Spiner (Cmdr. Data on Star Trek: The Next Generation - he "played" the violin on a handful of episodes, mainly those that aired after 1990)
Patrick Stewart (Capt. Jean-Luc Picard on Star Trek: The Next Generation - he "played" what is known as the Ressikan Flute (the actual player is Brice Martin on pennywhistle) on a couple episodes, most notably "The Inner Light")
~Ben