SM125 has sound. I've tried Ishar (one of my my few 'hi-Res' compatible games).... and it has the intro music and ingame sounds. But it's mono.
On my SM125 the sound level is set by the front 'lower' wheel (under the screen). It actually has a 'contrast' icon but set sound.
So no way to set the contrast, but IMAO it's already perfect.