Lifeclever writes that this is usually due to the way microphones and speakers are positioned on laptops, easily remedied by changing the audio balance.
The microphone [in the Macbook Pro] is located under the left speaker grille, and can contribute to horrible feedback during a call (the person on the other side hears himself coming out of the speaker on your machine, fedback into your mic). I changed my speaker balance so that output was all to the right speaker, and the effect was fantastic.
Though the author uses the Macbook Pro as an example, the concept works for any laptop. Also, I’d recommend checking your network connection, or moving closer to your wireless router if the connection gets fuzzy. [via Lifehacker]