Wow this thread got a lot of responses.
To my surprise...I knew this forum was thirsty for idle chit-chat.
I think we have to distinguish between
1) A listener who isn't paying attention, someone with ADD or someone who just won't focus when clearly & in good-volume is being spoken to. They might say "What?" because they neglected to listen to what was being spoken to them. This is a "decoder" issue.
OR
2) A speaker who is mumbling under their breath, speaking at a volume level that has no chance of carrying further then 4" from their mouth (we'll go 9" for Big Tom

) and basically someone who just doesn't speak loud enough for the words to audibly fall upon another person's ears in a manner that they can actually have a chance to hear what is being said. This is an encoder issue AND the topic of this thread: people who dont' speak loud enough.
Our nanny is the worst, not only does she mumble and dribble her ill-pronounced words, she actually doesn't even finish her sentences or she walks away mumbling.

Here's a technique I use when dealing with poor listeners....and trust me, having a wife, 3 young daughters and an idiot nanny, I've had to develop these skills!!
Step#1 Begin addressing your subject in a clear, loud & legible tone.
Step#2 Continue to speak at your subject until he/she gives you some acknowledgement that they are aware you are speaking to them. Once you know they are hearing you...
Step#3 Begin again at the beginning of what you were saying.
Sure, you could just as well ask if they can hear you and then begin to speak, but by re-starting what you were saying it has a greater chance of them hearing it (familiarity) and it rather conveys an annoying attitude at them to make them realize you are having to deal with their inadequacies as they are poor listeners.
Allow me to demonstrate, addressing my nanny:
Approach & face nanny, "Aviva, the kids are coming home at 3pm and they'll need to have a snack before they are......."
Aviva then looks up at me, realizing I'm talking to her, I then re-start:
"Aviva, the kids are coming home at 3pm and they'll need to have a snack before they are picked-up for ballet class".
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When dealing with mumblers and those who can't engage their vocal cords & lungs in sync.
There are several techniques, but the thing to remember is that in order for these inept people to learn, they have to incur an annoyance for their having done something wrong. When they mumble or speak softly, here are your response options, in the order of intensity:
I'm sorry, I know you'll find this hard to believe, but I just can't discern your words on account of them being mumbled in a sub-audible tone....
What, #1,217 <-- you have to keep count of how many times you say what, kinda tough but worth it.
Oh, I'm sorry...I'm fluent in English, I speak a bit of Spanish and at times I've even understood Italian, but I'm afraid I'm not fluent in mumble, could you please translate that into something I and others can actually understand?. You can also abbreviate this as
"I'm sorry, I don't speak mumble....you'll have to repeat that"
Ignore them, no response at all. BUT be careful, later on they'll do something that pisses you off and they'll tell you they told you about it, etc! Example
"Remember, we're going to my mom's tonight for dinner, I told you!" So you have a short-term gain with a long-term risk potential, use this one sparingly!
Oh, I'm sorry...my hearing's been tested and while I'm above normal, I just can't seem to hear what you're saying, do you suppose you could repeat that in more of an audible manner such that your words actually have a chance to make it to my ears with some semblance of reasonble volume & clarity?
And then there's the blunt approach.
Would you ****ing talk loud enough so I can hear you, I swear to God!