There are various reasons why people mumble when they talk. Sometimes, they mumble because they are too nervous to say how they feel in a louder or sterner voice. Also, people mumble when they talk while saying something that could come across as offensive or disrespectful. This is usually referred to as 'talking under your breath.' When a person does this, it generally means that he or she wants to express a thought or feeling, but he or she knows the words could hurt or anger someone else. Another reason people mumble when they talk is because they are unsure of how to articulate their words because they are having trouble expressing themselves, or finding the right words to explain something.