Most songs don't have different L and R channels, but using only one without combining them can mess with the song.
A lot of music has different L/R, but professionally made tracks are designed to sound the same (or at least close to the same) when reduced to mono. Bass and vocals tend to be mono already.
Does this create one combined channel or two different ones?
I'm not 100% on the context in Audacity, but if a song is in stereo then it already has two channels. I think Audacity splits them into two waveforms that are treated as separate.
If you want a mono track, then try using 'Tracks > Stereo to Mono'.