External speakers make buzzing noise when laptop is plugged in

Sounds like a power supply issue. In a computer's power supply, a constant voltage is maintained by a component that switches the power to a storage capacitor on and off very rapidly.

I'm willing to bet that what you're hearing is the voltage regulator switching on and off. When you do anything that requires additional power (i.e. turning up the brightness up, or running the CPU at 100%) the regulator is kept on for a longer period during the cycle. When you use less power, the regulator "on" time is shortened. This switching is generating electrical noise which is being amplified by your speakers.

Power supply manufacturers go to great lengths to minimize the electrical noise generated. It's possible that your power supply may be faulty. Do you know anyone else with a Thinkpad? If so, see if you can reproduce the issue when your laptop is plugged into their power supply. If it's still happening, it could be an issue with the motherboard. If the ferrite core has become detached from the end of your laptop's power cable, that could also be the cause.

There's a chance that a driver update could resolve the issue. It's a small chance, but worth a try. Assuming you're running Windows, download and run Lenovo's ThinkVantage System Update utility and install any available audio driver updates.

You need to put about a 50 ohm resistor between Left speaker wire and ground and another one between the right speaker wire and ground. I just spliced it directly into the cable going to my speakers. Some speaker systems (like mine) have a high impedance inputs. If the cables (or inside the computer) is not shielded properly you will get power supply buzz. For some unknown reason Dell disconnects these lines inside the computer sometimes when the sound card is not being used. The resistors give the speaker a path to ground for any noise that is introduced. The speaker/headphone jack is made to drive about 8 ohms, so 50 ohms will not load it. I could not find a cable available like this so I built one and it worked great. If none of this makes any sense, take this write-up to a techie with a soldering iron and they can make you a cable.