Mumbling would be one of a range of verbal communication issues. Mumbling, waffling and indeterminate noise responses to say 'yes', 'no' or simply 'hello' come to mind.
I have always been softly spoken and speak very deliberately and often at length if allowed. I often have people who tell me to speak up. Others simply aren't listening anyway and cut me off at some point and continue along a separate line of discussion. I have made conscious efforts to speak loudly and more deliberately but even when I think I'm shouting, its not enough (apparently). My daughter often says to me that I take a long time to say anything.
My own perception of all this is that people in general do not think rationally or systematically about things in general and are far too driven by emotionally charged 'point-scoring' and 'sound bites'.
I have ongoing problems with acknowledging what a person has said. To acknowledge what a person has said, I make this sound (a sort of low level grunt). It is understood to mean that I want them to repeat what they are saying because I didn't hear them.
Most people seem to be able to effortlessly make gestures and say 'hello' or 'goodbye' by means of their own unique vernacular slang that is entirely understood and socially accepted. In my case, when (for example) I pass by the receptionist in the morning, I say something like 'Morning' or 'Cya' but it inevitably sounds like a bird chirp or as if I'm in a bad mood. This issue is an ongoing problem until people get to know me and understand my particular communication'style' (if it can be called that) and make allowances for it.
While people say I mumble, speak too softly and waffle, I similarly have to often stop people and ask them to repeat what they are saying because I did not hear them or did not understand what they are saying. As a result people often think I am partially deaf which is not the case. Its quite clear that ASD and NT people are on very different audio frequencies.
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