You can’t put the mp2 audio in place of the original audio without re-authoring the dvd. Also, editing the video is going to do some strange things to the timing, so the original IFO files shouldn’t work with it properly.
From the bitrate, it looks like the original audio is full rate DTS audio.
I’m not quite sure why you are converting the original audio to MP2, unless you just want to save space. Perhaps Premiere won’t output as DTS?
You should be able to export from Premiere as mpeg2 in dvd compliant specifications. At that point all you need is an authoring program that won’t re-encode the video and give you a full dvd-video as output. If Premier won’t play nice with the original sound and you are relying on this (ugh!) Xilisoft program to convert, I’d suggest another tool.
Take the output from Premier and run it through AVStoDVD. The output will be a finished dvd ready to burn. Make certain to set it to output in the size you need, (DVD5 or DVD9), PAL specifications, and I’d use AC3 audio instead of MP2, but that is your choice. If your video output from Premier is actually dvd compliant, you can set AVStoDVD not to reencode the video. The audio can done that way as well, but it depends on what you’re getting from Premiere.
AVStoDVD is free to use: http://www.videohelp.com/tools/AVStoDVD
AVStoDVD’s weakest section is its menu creation, but you don’t have to make a menu if you don’t want to. The basic ones will work ok if you do need one.
If you are trying to retain the original menu, you’ve got problems, and things get much more complex.