I use OBS Studio to record videos, and would ideally like to record multiple audio tracks (so that I might balance audio levels in post-production). Unfortunately, when I import these MP4 videos into Sony Movie Studio Platinum, it detects no audio tracks.
The problem is a buggy DLL file.
To fix this, go to your program's install folder (mine resides in D:\Applications\Sony\Movie Studio Platinum 12.0) and find a subdirectory called "FileIO Plug-Ins\compoundplug". In this folder, you will find a file called "compoundplug.dll". Simply rename this (to "compoundplug.backup", for example) and your Sony Vegas or Sony Movie Studio should now import mp4 files correctly - even with multiple audio tracks.
Update, 2017-08-03: Never versions of Movie Studio likely do not have this problem. I have upgraded to Vegas Pro 14, and it reads files with multiple audio tracks just fine.