Is there a way to get a fairly constant sound volume throughout different scenarios?
For some apps, I have to increase the sound volume to hear it.
Then when I reboot, the volume is way up and startles me.
I use this, but I would like something better.
#!/bin/bash
#
# Set volume to designated percentage
#
amixer -D pulse sset Master 20%
I hope I mad sense.
As a mechanism to implement the solution (not a fix to your problem), you could try doing like @pavlos suggested to me in this.
My own case was to create the following script (example of /etc/rc.local), which contains anything you need started after the basic OS has finished booting (even before you open your GUI session):
#!/bin/bash
/DB001_F2/Oasis/bin/HW_Admin__Power_SetFreqCPU.sh --default --service
As you can see, my file only contains the 1 line ... for now! 
------ edit ------
Reviewing these two references,
I think the command you have proposed is correct, as a boot-time solution, but each application has its own override, IF their code logic ignores the computer's current settings which, unfortunately, some do! But you won't discover those until you go to open those Apps.
Eric. My volume scripts work at a desktop.
rc,local is a text document, not a directory .
"rc.local" is a script to be run by the OS at system start, before you actually log onto your computer.
I just tried the command, at command line, and there is no need for GUI, so all is good to include that in an rc.local script.
There would be absolutely no harm in trying your script in the rc.local . If it gives you what you want, one less thing to do manually.