If there is a word for "to mumble" in Spanish, I have yet to come across it. Sure, there is "hablar entre dientes," which means roughly "to speak without opening your mouth" and you can say that someone "no modula" (doesn't enunciate) of that they "habla muy cerrado" (speaks with a closed mouth) or pa' dentro (that they don't project), but that mumble-jumble, that word that you get to tell people to stop doing? It just doesn't exist.
Which is really a shame. Even though (and perhaps because) I've been speaking Spanish for years, I still get stressed out when someone speaks and I don't quite get what they're saying. It is an affront to my linguistic geekery and full-of-my-selfness in the language department to have to say, "what?" (cómo? qué fue? perdón? qué dijiste?)
Now, the lack of a direct translation for "to mumble" does not prevent people from doing it. There's the regular Chilean "eating" (swallowing) of ds, ts and s's depending on where in a word they occur, and then there are these people who talk alot like the adults in the Peanuts, only more quietly. And I can't understand what they're saying.
Which is another issue. For some reason that may be explained by physics, or may just be a complete misappropriation of a word, Chileans tend to describe someone who does not enunciate, and speaks very quietly as speaking "despacio."
You would think that despacio means "quietly." In fact, it means "slowly." In fact, one of the first phrases you learn in Spanish, right after "Dónde está el baño?," is "Puedes repetirlo?" (can you repeat that? which I would actually say "Me lo podrías repetir por favor?"), and then comes "Mas despacio, por favor." Slow it down, buster.
And certainly everyone understands that when you want someone to speak "mas despacio," it's that you want them to slow it down, not to speak more quietly, which when asking someone to lower their voice, you say that, "puedes hablar mas bajo? por favor."
I'm supposed to be a descriptivist, that is, a person who describes what people say, rather than a prescriptivist, who tells them how they should talk. It's a division among grammarians, a separating of the extreme geeks from the super geeks, if you will. So it should not bother me that people say "despacio" when they mean to say "bajo." I should note it in my anthropological linguistic notebook and move on.
But I hear this in my head all day long. Despacio, despacio, despacito (very slowly!) And I can either write about it or puzzle about it as I should be working on something else. And if my mind would just mumble, I wouldn't hear it so clearly, or maybe if it spoke lower (or even more slowly), or between its little mind-teeth, it wouldn't bother me so much.
But it does. And now it can bother you, too.