I'm actually clairaudient as well. It works differently for different people, but there are various exercises that you can do to get used to it. The following one is pulled from "The Element Encyclopedia of Ghosts and Hauntings"-- a fantastic reference for information on virtually anything occult-based, by the way. In particular, it's from page 86.
"Find a place where you feel comfortable and won't be disturbed. Sit down, relax, and breathe deeply. Imagine a shield of golden light around your ears for hearing and protection. Next, imagine a tuning knob such as one a radio in front of you. With this knob you can tune your hearing to a new frequency-- that of your higher awareness-- where you can receive clairaudiant guidance. Sense a subtle sound change as you adjust the frequency. Note whatever information you receive. You may receive very little at first, but keep trying, and after a while you might find that your ears automatically tune to receive clairaudiant information whenever you focus."
Keep in mind that this may not work right away or at all, but it's the one I used for a while before I got to the point I'm at. This particular book has a great definition for claidaudience too, but it's a bit lengthy to type out. Also, the mumbling you heard could very well be a spirit guide; sometimes they can sound a bit garbled until you practice listening to them.
I hope this is helpful. Feel free to message me with any questions or anything.