Share your experience!
The volume level for text notifications is much lower than my ringtone.
As an experiment I set the same mp3 as the ringtone and text notification sound. This confirmed that the text notification I am receiving is significantly lower than my ringtone which is set to maximum.
How can I increase this volume level?
I have had the volume issue myself for a quiet alarm but found a fix
go into your setting/sound/sound effects/sound enhancements click settings tab and check off xLOUD this will enhance the loudness of the internal speaker....I do hope this helps 
I've had my Z2 for 4 days and I too have noticed this LOW tones and notification vlolume that is a BIG PROBLEM!!
I hate it so much because I misse many text messages and reminders. It's liek teh vloume drops 50% whe I receive any text message, but when I pick a tone frome the setting, they all sound nice and loud. What the **bleep** is going on.
The other problem I found with the notifications, is that on the lock screen (or when my screen is off) MANY times there is no sound at all.. and some times its a dinky little sound that I can abrely hear even when in my hand!
WHat the **bleep** is wrong here SONY? Why won't you fix this major problem.
I plan to retunr my phone tomorrow. I had an iphone and it worke greatm it was just an old i4. So dissaponted with this new so called "flag-ship" phone!!!!!!!!!!!!!!!!!!!!:smileyshame_on_you: 
And the batter issues... don't get me started. So far it take 3.6 hours to charge my new phone!!!!
In 30 minutes I only get 12% charge. this is just wrong! I'm so angry!!!!
Same problem for me as well. All sound notifications can barely be heard. As you guys said, when you go into settings and want to choose the notification sound, when you play it from the settings menu, it sounds great, volume is just as it should be. But after that, when you receive any type of notification mail/sms/etc the sound is just too low.
My phone batch is 14w24.
Hope Sony will solve this problem in the next update!
xLOUD is no real solution since this distorts the speakers in other app and is deactived on prupose on my phone because of that.
+1, it happens to me too in my new Z2.
I come from an Xperia L so...it´s not in all Sony devices.