Audio recordings are used widely as crucial evidence due to the popularity of smartphones. However, the digital evidence submitted voluntarily may be questioned for its authentication. Since most smartphones provide the function of digital editing files, they could be exposed to crime as being edited maliciously. Primarily, it could be severe in the case of voluntary submission via information and communications networks. This paper proposed the forensic method for authentication of audio recordings generated by the Voice Recorder application from Samsung smartphones. First, we analyzed and compared the file structures and timestamps of audio recordings between the normal state without manipulation and the manipulated state. Then, we analyzed the spectrogram of audio recordings. Finally, by analyzing the timestamps of the media-log for audio recordings, we confirmed the authentication method for the audio recordings forensically.