You know what I kind of suspect it is, because they're almost 7-8 years old and they've been abused quite hard. But the volume dial is analogue so I don't understand how?
I've seen it happen to some speakers with analogue volume dials when I was studying computers and electronics at a school.
The teacher simply said that electronics work in wierd ways and told us to throw it in the garbage instead of fixing it.
(He couldn't be bothered to order a new volume dial. )
It could also be a internal power issue, it might suddenly stop working or start spewing smoke if that is the issue.
I have been having the same thing happen on my computer for the past month or so. Im on WIN 7 though. Mine started when my nephews stayed here for a week and were downloading a bunch of Skyrim Mods. I figured it was a virus or something, but nothing came up with Malwarebytes.
Its doesnt do it constantly, but like the OP said, seems completely random. Staying tuned in to see if anyone has any ideas.
Try a different cable, or moving it around. May be small short. I have an issue familiar to this with my amp for my music production PC. Its has a cross-over switch thats dusty, and if it isn't sitting PERFECT, it will cause my left channel to fade in and out.
Almost to the point where I'm going to solder the joints and make it full range permanently. Sometimes i need to run LPF (bass only) for making music and getting tone and notes right, so I REALLY don't want to permanently solder it.
Personal best - 11,811 in 3DMark06 on 01/16/2016 socket 939 baby!
I am now almost sure that it's the speakers fault, because I don't have problems with my Roccat Kave USB headset, will update if it starts happening with them. And that will most likely mean that it's some kind of annoying malware.