I am sure it is a software issue..
Volume is pretty high with songs, and when u go to settings > sounds to choose a notification and u play it, it is also high.. But when u set the notification sound it is just very low and u barely hear it.. They said it is solved on the new 314 update, but I can see it the same.. In contrary with the new update, I stopped getting updates in Sony apps, battery is draining faster, 4k recording stops in few minutes due to overheating problem.. What bothers me the most that I can't hear my phone unless it is very quite and next to me.. But when I compare it to other Android devices, they are much louder.. For an example, whatsapp default notifications.. I don't hear them while I am using it unless I focus, but for my sisters iPhone, I hear them from next room and she even got surprised thinking her phone is not that loud.. Volume on my Z2 is set to max but on her iPhone it was not and it is still much louder