Finally able to correctly convert USM file without any artifacts.
01. Demux USM file using VGMToolbox.
>>>> This will produce multiple files depending upon which USM file used.
>>>> One file will be the video file in M2V format.
>>>> One audio file containing music only.
>>>> Other will be audio files containing spoken languages. English, spanish,...
2A. Convert Video (M2V file) using Mediacoder
2B. Convert ADX Audio files (Mediacoder will do this also)
03. Remux files (MP4boxer or mkvmerge or whatever is your preference.)
I have tested this on serveral Witcher 2 USM files and the conversion process works without any artifacts.
Special thanks to all the developers for their great effort here.
can anyone provide some details instructions. Demuxing and converting was fine but both mkvmerge or mp4box gave me error while remux them back.