Audio authentication is important in content delivery via untrusted intermediaries, for example peer-to-peer (P2P) file sharing. Many differently encoded versions of the original audio might exist. Distinguishing the legitimate diversity of encodings from malicious tampering is the challenge addressed in this paper. We develop an approach based on distributed source coding for the problem of backward-compatible audio authentication. The key idea is to provide a Slepian-Wolf encoded quantized perceptually significant audio projection as authentication data. This version can be correctly decoded only with the help of authentic audio as side information. Distributed source coding provides the desired robustness against legitimate encoding variations, while detecting illegitimate modification. We demonstrate reliable authentication at a Slepian-Wolf bitrate of less than 100 bit/s.