I hade xenial-proposed updates activated and I started to have some troubles with evince. I deactivated the -proposed updates and I just followed the instructions of
How can I revert back from an upgrade to the Proposed repository?
to downgrade to the xenial stable configurations and updates.
After rebooting my audio crackles ! I don't hear any sound, just crackles, even when ubuntu starts.
I followed the instructions on https://help.ubuntu.com/community/SoundTroubleshooting and everything seems fine, and now I don't know what to do.