First check with Audacity in the spectrogram:

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