GRS1,

You may have to a bit of trial and error fitting but this procedure is quite simple.  I find it easier to match wave patterns than listening to the sounds.  I'd suggest not removing the camera audio as it will help with the syncing -- you can mute it later.

First have your camera video/audio on tracks one and two as usual and then place the mp3 audio on track three.

If the wave forms are not displayed do a right click on the each audio tracks and create it.  Expand the display with a vertical zoom (using the [+] icon at the right side of the timelines) to see the matching patterns of the waves more easily. Place the playback cursor at a distinctive point and zoom in (using [+] icon at the bottom of the timelines) to make it easy to slide the mp3 audio left or right so the wave patterns of the two audio tracks line up exactly.

Do a match very near the beginning of your clip and then check at the end to see if things are still in sync. If the points don't match you can use the mouse 'stretch mode' ( <-> icon above the timelines) to extend or contract the playing time of the mp3 without changing the pitch until you are in sync throughout.

Check how the clip plays using the new audio and do a further adjustment if needed. If the camera and mp3 recorder aren't running a constant speeds you might have to divide the clip and do the syncing a section at a time.

Regards,

Carl