Yeah, I'd say that mumbling could be considered a symptom of shyness.
I mumble too apparently. My name is Laura and people always think I'm saying Maura! It just happened yesterday in fact. I had a meeting with someone and went up to the receptionist to check in and she asked my name and she repeated when I said and thought that I had said Maura. I corrected her, but some of the times I'm so embarrassed that I won't correct them and will just let them call me Maura the whole time.