This has been getting a lot of hits, so clearly, it's a common problem! If this advice helps you, or does not help you, I'd appreciate it if you could leave a comment so I can see if the instructions need to be clearer (ie, more pictures and whatnot).
This is 100% unrelated to anything else on this blog, but I am submitting it on the off chance that my recent experience can help someone else out there. Like a public service announcement! I did a few Google searches to help me solve this problem and the web was full of bad advice, although eventually I did find the right solution, which I present below.
Connected laptop to TV via RGB cable and simple audio cable. Video came in fine. Audio was mostly fine, except for one problem....
The problem: When laptop was plugged into electricity, the TV speakers made a very annoying buzzing noise. When running off battery, no such noise. Laptop is old, and battery lasts less than an hour, requiring it to be plugged in. What an odd problem to have!
The good news is that there is an easy solution.
Technical name for problem: Electrical interference caused by loop in grounding wires. Basically, when the laptop was plugged in, and the TV/speakers were plugged in, they managed to interfere with each other in some magic electrical way.
Solution: 50 cent three prong to two prong adapter.
Purchased this little thing at the hardware store. 2 for 99 cents.
Three prongs go in one side, two prongs come out the other.
Simply plugged laptop into adapter and then adapter into wall.
Buzzing noise has been vanished forever. Hooray!
So if you have this problem, try this very simple solution!
Further advice if the problem persists:
Note that the adapter has a little metal loop which tries to make contact with a screw in the outlet and act as a grounding device. This will obviously cause the problem to return. Either bend that piece of metal back or remove if it makes contact and the problem returns.
If the problem still exists, try using a second adapter on the TV or speaker system as well to avoid grounding those and encountering interference.