Losing audio on video mixdowns in Audition?

I have Audition 1.5 and am making personal edits to music videos using high quality audio files and muting the original music. When I export the video and playback the saved file the audio is a horrible low quality sounding bitrate. I have tried multiple different codecs but all sound low quality. Is there a different way to save? Or am I missing something?

Or is there an easier software to use? Audition is my favorite audio editing software and I would assume there is a way to save this video with the quality at full bitrate.

Any help is appreciated!

What is the source of the original videos, DVD, VHS, downloaded avi or mpg or flv files? What do you mean by “high quality audio files”, mp3, wav, pcm, etc? What is the source of the new audio files. What program are you using for the video processing? You say you are muting the original music, how are you muxing the new audio/video?