They didn't realize that every human voice has its own private wave pattern. Private as a fingerprint.
9 seconds sound clip from Batman (1966) - He Meets His Match, the Grisly Ghoul (S1E16) TV Series.
You can hear this line at 00:05:25 in the Blu-ray version of the movie.
Quote context
[...]
- Before setting out tonight, I equipped my utility belt with a tiny microphone and transmitter.
- Every word those criminals said was recorded in the Batmobile.
- But what good will that do, Boy wonder?
- I thought you said they spoke to you through a trick voice distorter.
- Tricky, commissioner. Tricky, but vain.
- Astounding. Is it evidence? Can it be used by the district attorney?
- True, the courts won't accept it as yet.
- That way, we should be able to learn the identity of the Joker's henchmen.
- We might be able to follow one of them to the Joker's hideout.
- Robin's right. It's our only card. Let's hope it's a trump.
- I'm afraid the Joker may still hold the winning hand, Batman.
[...]