It sounds like your original stereo file is out of phase. This typically occurs with recording equipment the uses mini jack connections. You will not notice that the recording is out os phase before you hear it in mono. When the left and right channel are mixed together the bad phase will result in the sound being nearly totally canceled out … at least all the sound that the two channels have in common.
The solution is to flip the phase on one channel - or split the stereo channel and only use the one side.
- Select the corrupt file.
- Goto the menubar.
- Tools / Split stereo
- Delete the one channel.
You can use different VST tools to switch the phase, like the STTool.