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:
This looks like HEX due to the characters used, which converts to this string:
Now this looks like BASE64, so a quick decode reveals the 2nd part:
If we put the two together, we get the flag:
Credit: Jonathan Singer