I read on some forums that a “new” virus W32.Sober.K@mm (Sober.L) is spreading massively,…but I didn’t found this one in the viruslibrary off Avast! :-\

Different av vendors gives different names to malware.

eg:
I believe Sober.K is known to Avast as Sober.J

How do they come to a name like “Sober”,…is it mentioned in the code of the virus??

You should ask the one who discovered it.