Cant compile many sources cause i dont have autoconf.h in /usr/src/linux/include/linux/. How can i get this file?

well if you are missing a file and don't know in which package it relies just do a check on Debian packet search contents and this would have got you:So do a apt-get install libc6-dev and you're set :-)

ak:~# apt-get install libc6-dev
Reading Package Lists... Done
Building Dependency Tree... Done
Sorry, libc6-dev is already the newest version.
You might want to run `apt-get -f install' to correct these:
Sorry, but the following packages have unmet dependencies:
libvorbis0a: Depends: libc6 (>= 2.3.1-1) but 2.2.5-11.2 is to be installed
libvorbisfile3: Depends: libc6 (>= 2.3.1-1) but 2.2.5-11.2 is to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

I tried to get .ogg files playing sometime ago.. Do i dare to force?

Looks like you tried installing something from unstable? If you want to do that you need to backport the package, which can be a very trival process.

Do: dpkg --purge libvorbis0a libvorbisfile3

afterwards: apt-get update && apt-get-upgrade might be a good idea ...

ok that helped. it says my libc6-dev is newest version tho. how can i reinstall it?

which debian version are you using?
cat /etc/debian_version

if you are using < 3.0 consider upgrading:
edit /etc/apt/sources.list and add
deb http://ftp.de.debian.org/debian/ stable main
deb http://non-us.debian.org/debian-non-US stable/non-US main
deb http://security.debian.org/ stable/updates main

apt-get update && apt-get dist-upgrade

if you are on 3.0 already try to locate the file:
find / | grep autoconf.h

if that doesn't help:
apt-get install --reinstall libc6-dev

for more details: man apt-get