To mumble is to talk quietly and indistinctly. When people speak in a mumble, it's hard to understand them.
Mumbling is a way of speaking that’s a little like whispering. Like whispering, mumbling is usually done with a low voice. However, mumbling is harder to understand than a whisper. When we mumble, we don't form out words clearly. Mumbling is the opposite of speaking clearly. Some people mumble because they lack confidence. Others might mumble because they're tired or not feeling well. If you want to be a good public speaker, you'd better learn to not mumble.