This simple circuit can be used to control the volume of PC speaker. The volume can be adjusted to any value between total silence and full volume.
---------------+
|
From | |470 ohm linear
motherboard | |<----------------
Speaker connector |_| To speaker
|
---------------+------------------
The circuit has to be built inside your PC so be careful what you do. The hardest part in making this circuit might be to find a good place in PC case for the volume know and connecting it to that place.