What would be the auditory equivalent of indecipherable?

indecipherable would be an OK word to use, in the broad sense of decipher as “to succeed in understanding” (Oxford American Dictionary).

But I get what you’re saying. “Unintelligible” would work, and that’s probably your best bet, though it’s not specifically limited to sound.

If you want a word that specifically refers to sound, try “garbled,” though that’s not going to work as a direct replacement for indecipherable.

inaudible or muffled

Going completely over the top here but…

Inarticulate, incognizable, incoherent, incomprehensible, indecipherable, indistinct, inexplicit, jumbled, muddled, obscure, unclear, opaque, vague and unintelligible. (personally, I’d use unintelligible)

Unintelligible is the word used in court transcripts and in transcriptions of recorded interviews, etc., when the person speaking cannot be understood (for whatever reason, including mumbling (indistinct) or ambient or mechanical background noise (muffled, garbled). So under current usage it’s best.

Unintelligible. Much like what goes on between my ears.