Hey Magix, I was just editing a video where I review a mod for minecraft, BUT in Vegas 15 the audio is super loud and it hurts my ears. Here is proof that this happens 
Try remuxing your MP4 files.
Original Post: https://www.vegascreativesoftware.info/us/forum/new-avc-issues-in-vp15-try-disabling-so4compoundplug-dll--108345/
If the troublesome files are getting decoded by so4compoundplug.dll then you can try disabling it as follows:
1.Hold down the SHIFT key while opening Options > Preferences
2.Click the Internal tab
3.In the field "Show only pref containing:" file type "So4" without the quotes
4.Set "Enable So4 Compound Reader for AVC/M2TS" to FALSE.
5.OK > Restart
With luck, VEGAS will now instead attempt use an alternative decoder such as compoundplug.dll to decode your files, which may solve the issue.
Bear in mind that this may break some of the new support such as iPhone and 10-bit 422 GH5 files, and that the bugs with so4compoundplug.dll will probably be fixed in future updates, so don't forget that you've set this!