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For this 29c3 CTF challenge, we are given a WAV. Upon first listening, one can hear DTMF tones, phone tones. There is still an abundance of high pitched noise too.

First check with Audacity in the spectrogram:

Highandlow 100

Now we have the first part: 29C3_THIS_IS_ONLY_HALF_

From there, we need to determine the DTMF tone codes. There are many software solutions that can complete this.

The tones come out to:
644768706331397063313930614756666233526f5A584A66614746735A673D3D

This looks like HEX due to the characters used, which converts to this string:
dGhpc19pc190aGVfb3RoZXJfaGFsZg==

Now this looks like BASE64, so a quick decode reveals the 2nd part:
this_is_the_other_half

If we put the two together, we get the flag:
29C3_THIS_IS_ONLY_HALF_this_is_the_other_half

Credit: Jonathan Singer