The Sony Xperia Z2 is a phenomenal flagship smartphone. The camera is outstanding, the battery life and water-proofing are great, and it's one of the few devices on the market to sport front-facing stereo speakers. The speakers are loud and crisp, but the stock firmware doesn't allow for notifications to be heard through both speakers. This leads to quieter notification sounds and potentially missed notifications. If you can't hear your phone, what's the use of the front-facing speakers?
Don't worry. As is usually the case, a great dev over on XDA has come to your rescue. XDA user olokos has tweaked the sound system files to allow for the notification audio to be played out of both speakers, not just the bottom one. olokos also increased the volume output. Not only will your device play audio from both speakers, it'll do so much louder than before.
You'll need to be rooted and running a custom recovery to use this modification for your Z2. Head over to the XDA thread and download the files listed, then reboot into recovery and flash them. There's a sound mod that also boosts your headphone output, one that tweaks the speakers, but doesn't boost headphone output, and then a file to revert back to the standard KitKat sound files if you find that you don't like the mod. The dev is also working on a few more sound modifications for the Z2. A speaker boost mod is coming soon, as is a tweak to boost Bluetooth audio volume.
And here's the standard disclaimer whenever you are tweaking your phone or other Android device: anything you do is at your own risk. In reading the thread for this mod, it appears that everything works. The mod isn't breaking anyone's Xperia Z2 or causing all of your data to be sent to hackers in China, but that doesn't mean you shouldn't be careful. Always backup your existing setup before you start flashing something new and make sure you have the revert files from the thread just in case something goes wrong. Have fun!