Thanks for the reply Nodsu.
No, I haven't turned the balance knob - there isn't one!
Let me explain a little more clearly. I'll call the track in question track A. When I play track A using Sound Forge 5.0, I hear sound in both speakers (Labtec, self-powered, stereo computer speakers - about 5 years old; no balance knob). I see left and right wave forms and note they are different. I see both signals represented on the VU meters. When I "solo" the left side, I hear no sound - nada! Meters are still moving, the wave form is still "waving", but there is no sound.
I then copied the left track and pasted it into a new file (track B) and bingo, I can hear it! Now if I copy the right side in track A and mix it with track B, I hear both sides but I now have a mono signal. Does this phenomenon have anything to do with recording/encoding MP3 files? Is there something I need to do before playing back this file?