how do you have your speakers connected?
and is any audio coming out of the speakers?
Start Menu -> Control Panel (switch to classic view) -> Sounds & Audio Devices
you can select what type of speaker set up you have, and what audio line its coming from.
Line-Out port? - It should auto default out there, and should mute the internal speaker from that point.
Headphone Jack? -> this usually silences any other audio input device for the most part.
Optical? -> if you have optical youre going to wand to lower the Master Volume; double click on the volume icon in the task bar and dont mute it, just lower the volume all the way down as optical has its own volume control.