Bring your file into the media room, then drop it onto the voice or music track. Select (highlight) the track, then click the EDIT AUDIO button and click WAVE EDITOR.
Top-right of the window there are two buttons beneath the word "WaveEditor." Click the left "single channel" button.
In the W.E. display, click on the channel waveform that has the audio, then press the following keys: <ctrl-a> followed by <ctrl-c>.
At the top-right- click the "edit both channels" button.
Press the following keys: <ctrl-v>. This should paste the audio from the active channel into the blank channel.
Click the X, top-right, to exit. You will be prompted to save.