are you wanting to simply sync up new audio sources for videos that have (lesser quality) audio in the first place?
or are you wanting to do more "editing" type stuff? like mixing several video sources together, making "fancy" transitions, yada yada?
if you just want simple (i.e. set an IN point and an OUT point, be able to trim video a bit), then VirtualDub should do fine. only problem is - IMO - it doesn't like AVI type 2 files, which is what i capture my footage to so i can edit in Premiere. (and yes, i know there's a work-around for editing type 2 AVIs in VDub with the panasonic codec, but it's a shite codec in my experience, and unlike most all other DV codecs, degrades the quality of DV AVIs in a hurry.)
sorry for so many questions. i ask because "vid/aud muxing" usually refers to multiplexing the audio and video together, i.e. taking an audio stream (MPEG, AC3, WAV/PCM) and a video stream (MPEG) and combining them as .MPG or .VOB.
anyway, for video editing, i'd suggest Premiere or Vegas (never used Vegas, but many people swear by it).
for DVD authoring, i've heard TMPG DVD Author is good as well as DVDLab. (never used either one, but a lot of people i trade with use those two programs.) Ulead DVD Workshop is also fairly easy to use - mostly drag & drop - for taking audio/video sources and compiling them for DVD.