I don't have one but there may be a boost in the menu for Volume.
You can also use the equalizer and boost all bands or the
ones you hear, meaning not very low frequencies since the speakers, earphone transducers don't reproduce those, boost all the mid and medium high frequencies.
SOME Z3Cs are very quiet in Volume , like the one tested at GSM Arena and some are very loud, like this
one:
Sony Xperia Z3 Compact vs HTC One Mini 2:
http://youtu.be/Z8UxXtGDOcA
Go to about 16:40 in the Video and you will hear the
Z3 C is louder than HTC M one Mini .
Unfortunately-not ALL Z3 Cs are this loud, according to
my unscientific research before I buy one.
Regardless-I am amazed that a few like you and,GSM
Arena find the phone too quiet and others report opposite.
Youtube Videos are not accurate BUT with two phones side by side still an indication of relative Volumes.
I conclude that SOME Z3Cs are much lower in Volume.