Finally I made it to kill this $%&§ sound on boot and shutdown (sounds like a news-intro on TV).
It´s very simple, but to kill the mp3 files you have to root. Luckily this is no problem with the help of our friend noidremained.
So first this: How to root the x7
and then go to /system/media and kill/burn/delete/rename the files bootaudio.mp3 and shutaudio.mp3 (it is not hard to find, the only two mp3 files in the folder!)
Alternative: To prank someone you could replace it with a 10 hours nyan cat mp3, but maybe the device will crash