I can"t import another audio file (MP4).
The first Audio i have imported is a MP3 file, everything works, when i would like to import the second audio file, the windows block icon came (it is the red icon, which means, that you can"t do this in the moment), so i can"t import my second audio file.
I"ve tried to import a second MP3 file but it doesn"t work either.
Yes, you can. You simply cannot drag-n-drop it. This is a known bug that no one knows how to fix because it is not easily reproducible by developers. However, there are other ways to add things to the timeline besides drag-n-drop. Such as toolbar buttons and keyboard shortcuts. Learn them.