Xandra from Florence, Vermont wrote:
What is my obligation if I walk into or through a room where five people are engaged in lashon hara (derogatory speech) about another worker who is temporarily absent?
It is a Torah prohibition to listen to derogatory speech. Therefore, if you have no business in the room at the time, then it is a mitzvah to leave. And joining the group or expressing interest in what is being said is prohibited. If you need to be in the room for some reason, then you have to try to listen to as little as possible, and try not to accept any of it as fact.
Certain leniencies apply to listening without the intent of accepting. Mainly, if you think some benefit may come out of it for you for example, they are speaking about someone whom you are considering for a business partner or, alternatively, if by listening you think that you will be able to rectify some wrong, then it may be permitted to listen. Here too, youre only allowed to suspect that the information might be true in order to protect yourself, but youre not allowed to believe it as fact.
Sources: Sefer Chafetz Chaim, Hilchot Lashon Hara, Klal 6