Background noise can have a profound impact on the effectiveness of communication in several different ways.
Most obviously, it can simply prevent someone from hearing what’s being said. They may mishear and misunderstand the communication. This can can cause stress and anxiety, which in themselves are further barriers to effective communication.
Ten people who speak make more noise than ten thousand who are silent. Napoleon Bonaparte
On the part of the sender, focusing on speaking louder to overcome the background noise can take their attention away from the content of the communication, making it more difficult for recipients to understand them. It can also prevent them from picking up on feedback from the recipients, and from judging the effectiveness of their communication.
For all parties, it can also serve as a distraction, particularly if the noise includes other conversations.