It sounds like the nut that holds the volume pot in might just be loose. This can be accessed and tightened without opening the speakers up. Try this:
1) Pull the plastic volume knob off the volume pot. Depending on how tight it's on, you might be able to pull it off with just your fingers. If not, look for something fairly flat and preferably plastic with soft edges to pry it with.
2) Use a deep well 10mm socket (or socket driver) to tighten the nut around the volume pots shaft.
3) Press the plastic volume knob back over the volume pot's shaft, and retest.