In my video program which is cheap Magix ($50) if I convert to *.wmv I use lossless 44.1kHz - 16 bit which is the best. Although *.AVI should have no problem in Sonar, I can not attest to that, because I use the video program which has a mixer to adjust the volume when I import the output tracks from Sonar, and I can input them at 48kHz - 24bit to the video program and can make a *.AVI file (which is way bigger) with 48kHz-24bit video output. *.wmv can only be made 44.1kHz-16bit and lossless does not mean that it is true stereo, because it is a variable rate which is lossless, which sounds like stereo but really is not true stereo at a constant bitrate and sample rate. If you use that kind of output - constant intead of variable, then the video will be like an *.mp3 which is what it will be.
*.mp4 is not all that good for sound, and unless you have way better equipment perhaps taking the original movie and a way better video editing program, then that is about all you will get. *.AVI files are not compressed usually but if it is, maybe that is the problem, because in the video program I have no problem importing *.mov files either, so if you have a video editing program I would first see if you can change the video format and output it in a format Sonar will recognize and then import it into Sonar, and also just the audio can be made into a separate track in my program (where in a *.mov file it is listed video and audio as one track, then it can be separated into 2 tracks.
Export just the audio then perhaps and work on it in Sonar, and then input it back into movie editing program and with the mixer in the program, down (0) the original audio and by inputting the audio into Track 3 then of course raise the volume on that track to the level you want it at in the video, then export the video in the format you wish to view it in - but for youtube I have found out, usually *.avi files run better but way bigger than a *.wmv file but that is because utube to me sucks. I never can - oh well.
That's about it.
Of course the video part can be added stuff also if you know what you are doing (fade ins such as, etc.).