Q:

Fix microphone volume for a user

I need to fix the microphone volume for any user in Ubuntu 16. What I want is to fix the volume, say, to %40, and keep it at that value for all times. It must be changed by no user except root. How can I do such a config? And with what tool?
Thanks.

A:

All mixer controls of a card are managed with a single device node (/dev/snd/controlCx), so it is not possible to use file permissions for this.
However, you could program a separate tool that calls
snd_ctl_elem_lock() for the mixer control that no other program should
be able to change.

