I've used Premier to do several videos with narrations before and always use the "Track Volume" slider on my main audio channel (audio 1) to turn down/off the main video's audio.
Today I created two videos and did the same. When I play them in the editor's preview window, the track is properly muted. However when I "Share" them to a file the track is mixed in at full volume.
I've tried several different output formats (MPEG2, WMV) with a myriad of different settings, but the track always appears at normal volume. I've tried rebooting, copying to a new project, no love anywhere.
Muting the track volume in sceneline mode may not silence it.
A better solution is to switch to timeline mode. There you can either control the audio volume (by dragging down the yellow line that goes through the audio tracks) or eliminating it completely by selecting the clip or clips you want to mute, right-clicking and choosing Delete Audio.