Step 1: Get Audacity. Google it for the download.
Step 2: Open your stereo song in it.
Step 3: Click the drop-down box next to "Stereo" and click "Split Stereo Track.
Step 4: Click the x next to one of the tracks, it doesn't matter which one.
Step 5: Click the drop-down box next to the last one and click "Mono".
Step 6: go to File- Export as .ogg
There you go.