I just got done spending hours troubleshooting this because the buzzing got worse. I am using these speakers on one side of the room, with a 25 foot rca cable going across.
I found the buzzing was severe if it wasn't connected at the other end, and the projector it was plugged into was off.
So oddly my buzzing dimmed so long as I left everything on. I have it now plugged into a powered audio splitter and it's mostly gone. This little guy stays on all the time.
Now the buzzing is still there lightly, if my lights are on. Doesn't matter if the speakers are plugged into the same circuit or not.
I did more work and found that specifically it was a set of outdoor string lights being on that caused the buzzing. Because of where I ran the audio cable the wire along the wall it was about a foot from them. I changed them going along the ceiling in the middle of the room so it's 6' minimum from these outdoor string lights and now the buzz is almost unbearable. I can get rid of it completely by turning off my outdoor string lights. Maybe they are radio active? Should I buy different ones?
The issue is so small now that it doesn't matter. But I may replace these string lights and see what happens.