I'm still in the process of understanding the problem here, sorry...
In Tails, when I set the master volume from the system menu to 25% and
play an MP3 with Totem (the default media player), raising the volume of
Totem to 100% doesn't change my master volume. The volume of Totem is
relative to the master volume.
The same applies to YouTube from Tor Browser and Audacity and seems to
not reproduce what you're saying about media players having the power to
maximize the master volume and become dangerous.
Now, if I install Audacious in Tails and plays an MP3 with it, then
raising the volume of Audacious to 100% brings my master volume to 100%.
I guess that's what you're talking about.
So my conclusion for now is that, with the flat-volumes on, PulseAudio
allows applications to misbehave (= "blow your ears") but that well
configured applications, like Totem, Tor Browser, and Audacity don't
present this dangerous behavior.
If I'm right until now, then I think that we should check whether all
applications included in Tails respects the master volume and fix it if
we find some that don't.
While doing my research I also found that this seems to be a hot debate
on Internet, but at the same time, I was surprised to found little
information about it on the PulseAudio bug tracker. If this is such a
controversial behavior, I guess PulseAudio also have a strong opinion
about it themselves. But I could find it, apart from this
https://bugs.freedesktop.org/show_bug.cgi?id=57355#c2.
