The NEC's have a built in audio amp and have speakers available as an option for around $600 - $700. I am sure you can get something better sounding for a lot less than that if you don't mind not having the speakers directly attached to the sides of the monitor.
I bought a pair of Polk bookshelf speakers for $150 and hooked them up to my NEC 50MP2 and they sound pretty good. I get a decent amount of undistorted volume even though the amp is rated at only 7 watts/chan.
The volume, bass, treble, and balance can be adjusted via the remote control. It also has a mute button.