Debian Stretch
Audacity 2.1.2
Installed from Debian repos
Problem: Audacity uses default audio output which is Alsa. Alsa should be controlled through Pavu mixer. That is, both volume and output (internal, haedphone) should be adjustable through Pavu mixer.
It was not. The slider could not be moved for Audacity output, nor could a different output be selected. Pavu mixer was behaving erratically refreshing the slider about 10-20 times a second.
Cause: it was a conflict somewhere, which was solved by removing the ~/.asoundrc file. This file was a remnant of a previous installation. After upgrade from Jessie to Stretch either Alsa, pulse or Audacity choked on this. It might not even have been Audacity related, but since Audacity is the only application talking directly to Alsa.
jlinkels