Thanks for explanation @Brendan_
I've checked again today, unpacked only Gamedata/SoundTrackEditor, Music and Playlists (I'm lazy and like to use what is already there :-) ) and all seems to work. I don't need Soundtracks in root KSP directory.
Only one thing I don't like is this warning:
To eradicate it I copy from Soundtrack Editor archive KSP_x64_Data/libmpg123-0.dll to KSP/KSP_x64_Data (I run KSP_x64 on Windows) and I can add and hear mp3 with no problems. I'll check how it works for a longer period of time :-)
And I have great request for @pizzaoverhead - could you please rearrange all files/directories to be in Gamedata/SoundTrackEditor/*? If it's in root KSP directory it breaks CKAN spec compatibility (https://github.com/KSP-CKAN/CKAN/blob/master/Spec.md):
install_to: The target location where the matched file or directory should be installed.
Valid values for this entry are GameData, Ships, Ships/SPH(v1.12), Ships/VAB(v1.12), Ships/(v1.16), Ships/(v1.16), Tutorial, Scenarios (v1.14) and GameRoot (which should be used sparingly, if at all).
A path to a given subfolder location can be specified only under GameData (v1.2); for example: GameData/MyMod/Plugins. The client must check this path and abort the install if any attempts to traverse up directories are found (eg: GameData/../Example).
Subfolder paths under a matched directory will be preserved, but directories will only be created when installing to GameData, Tutorial, or Scenarios.
I know that maybe you don't care for CKAN, but there are users using it and really happy with it.
Many thanks for this mod and for your support.
Luke.