(Because I’m waiting on a dishwasher-repair person, and that sort of thing always makes me peevish.)
Peeve the first: It’s vocal cords, people, not vocal chords. It’s an easy mistake to make, given that cord and chord are homonyms, and given the association with sound-making and hence with music . . . but the items in question were named by anatomists, not musicians, and for the anatomical mind the notable thing would have been their physical structure. Wikipedia has some good pictures, which I’m not going to reproduce here because while interesting, they aren’t particularly handsome or appetizing.
Peeve the second: This one’s a bit more subtle. If you’ve got a character listening in on another character or characters talking about something, but the listener isn’t quite able to make out what’s being said, the conversation isn’t undecipherable or illegible.
Undecipherable and illegible are adjectives for something that is, or is meant to be, seen or read. Something that’s undecipherable is, taken literally, unable to be decrypted or decoded; by extension, it refers to something drawn or written or otherwise seen, the meaning of which cannot be determined. (You can have an undecipherable letter, or an undecipherable carved inscription, or — speaking metaphorically — an undecipherable expression.) Something that’s illegible is something written that cannot be read, such as an illegible signature (though not — because it isn’t written down — an illegible expression.)
If what you’re dealing with is something that is, or is meant to be, heard, the words you’re looking for are unintelligible (the listener can hear it, but not well enough to make much sense of it) or inaudible (the listener can’t hear it well enough, period.)
I run into this one oftener than you’d think, and it drives me batty.