Editing an audio track in an Mp4 video WITHOUT video data loss ?

[PeteV]

I want to change the soundtrack of an MP4.
But if I open it in, say, Sony Vegas Pro, when I render it again in the same format, there's quality loss because I start with a lossy source. ( right ?)
Is there a way to simply swap the sound of an Mp4 file with another without touching the video ?

If so can you guys suggest me software ?

Thanks in advance ! I love how kind you all are to newbies like me.

[manono]

I think MP4Box with a GUI can do what you want. Demux the audio and video, do to the audio what you like, and then remux. The video remains unchanged.

[PeteV]

Awesome sauce ! Thanks !