If your mono track is being fed to the master bus correctly, when panned dead centre, you should be seeing both left/right signals on your master bus meter, rising/falling exactly the same? Pan right, the right side of the meter on the master should only be showing, pan left, the left side of the meter. If that is happening, and your right speaker is still bad, then it's the signal either in or out of your sound card, or even your right speaker. If you suspect a speaker, swap them around. Does it change with the swap?
If you are only seeing the left signal rising/falling on the master bus meter with the pan dead centre, then it's your track. If that's it, then we can go from there.