Stereo audio files have left and right channels to be heard through different speakers. If your device is incapable of handling stereo audio, it is easy to convert a stereo file to mono in Audacity so that both channels can be merged and heard as one.
1) Click the down arrow on the track to open the menu.
2) In the menu that drops down, select Split Stereo to Mono.
3) Click X to delete one of the two tracks.
4) To save the new mono audio file, click File.
5) Click Export to save file in a specific format.
6) Select a format to export. In this case WAV.
7) Click Save to save with the desired filename.