Depois de executar sudo apt-get update , esta é a saída no terminal:
W: The repository 'http://ppa.launchpad.net/n-muench/vlc/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://ppa.launchpad.net/ubuntu-audio-dev/ppa/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: https://download.01.org/gfx/ubuntu/16.04/main/dists/xenial/InRelease: Signature by key 09D6EF97BFB38E916EF060E756A3DEF863961D39 uses weak digest algorithm (SHA1)
W: http://linux.dell.com/repo/community/deb/latest/Release.gpg: Signature by key 42550ABD1E80D7C1BC0BAD851285491434D8786F uses weak digest algorithm (SHA1)
E: Impossibile recuperare http://ppa.launchpad.net/n-muench/vlc/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Impossibile recuperare http://ppa.launchpad.net/ubuntu-audio-dev/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Impossibile scaricare alcuni file di indice: saranno ignorati o verranno usati quelli vecchi.
O que devo fazer para corrigi-los?