How can I stop mumbling?

Growing up you’re generally told that no question is a stupid question, and that it is actually good to ask questions. I tended to either ask apparently dumb question and/or get made fun of or called dumb or something because of the questions I asked.

Since then, I began mumbling – mostly when asking a question. I think its because I feel like it’s a stupid question and I just don’t want to hear how dumb it is, but when I do this it just causes me more embarrassment because then people can’t hear me and ask me to repeat myself multiple times.

Any tips on how can I stop doing this? :(

FYI – I am now in my twenties and working full-time, if it matters.