If someone talks too low, or mumbles, do you call that 'inaudible' or 'unintelligible'?

To put it more specifically:

• is inaudible reserved for when you can't hear anything, or does it also apply when you can't hear enough?
• is there a more specific word than unintelligible to mean 'acoustically unintelligible'?

Inaudible is used in typed transcripts of taped police interviews where I come from, and covers both cases: it means that the words spoken cannot be heard clearly.

Inaudible is used to describe a sound that is too soft to be heard. The reason why it is too soft is irrelevant. It also does not necessarily have to refer to a voice or speech, and, strictly speaking, implies that the sound cannot be heard at all.

As for alternatives to unintelligible, mutter might be a good choice. To emphasise the unintelligibility of a mutter, it can be described as an indistinct mutter or incoherent mutter or variants thereof.