In case someone comes across this question, below is the solution I provided in another post (based on the solution given in Inconsistent Sound Volume Ubuntu 20.04, which deserves credit).
Basically, you need to change flat-volumes = no to flat-volumes = yes. The values "true", "1" and "on" are equivalent to "yes", according to the manpage (https://manpages.ubuntu.com/manpages/focal/man5/pulse-daemon.conf.5.html):
For the settings that take a boolean argument the values true, yes, on
and 1 are equivalent, resp. false, no, off, 0.
Don't forged to uncomment the respective line by removing the ";" or "#":
The configuration file is a simple collection of variable
declarations. If the configuration file parser encounters either ; or # > it ignores the rest of the line until its end.
The configuration file is: /etc/pulse/daemon.conf
Other possible locations are: ~/.config/pulse/daemon.conf, ~/.config/pulse/daemon.conf.d/.conf and /etc/pulse/daemon.conf.d/.conf:
The PulseAudio sound server reads configuration directives from a
configuration file on startup. If the per-user file
~/.config/pulse/daemon.conf exists, it is used, otherwise the system
configuration file /etc/pulse/daemon.conf is used. In addition to
those main files, configuration directives can also be put in files
under directories ~/.config/pulse/daemon.conf.d/ and
According to the aforementioned post, rebooting is necessary for the change to take effect.
About the configuration option:
flat-volumes= Enable 'flat' volumes, i.e. where possible let the sink
volume equal the maximum of the volumes of the inputs connected to it.
Takes a boolean argument, defaults to no.