It sounds to me like your volume control rheostat in the speakers themselves may have finally bitten the dust. These volume control knobs on the speakers work entirely independently of any device driver and are an analog adjustment just like every speaker control knob you've ever used (or at least all the ones I've had experience with are).
My only quick recommendation would be to remove whatever cover (probably on the bottom of the speaker) you can that gives you access to the "innards" of the volume control and give them a quick spritz of either TV Tuner Cleaner (yes, you can still buy this at places like Radio Shack) or a deoxidizing spray like Caig DeOxIt or similar. Sometimes it's that the rheostat has become dirty over the years.
You can test out whether my theory is correct by plugging these speakers into something like the output jack on your smartphone and play music from there. Provided there is any output to the speakers from the device (that is, it can't be muted or else there's no signal to adjust) your speaker volume knob should be able to boost or reduce the volume there. I suspect you'll find it's not working on other devices, either.
Edited by britechguy, 12 December 2015 - 01:15 PM.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall