Notification sounds do not trigger about half the time

Ever since I've upgraded my iPhone 6 64GB to iOS 9 (through a tethered iTunes update, not OTA), I've been having an annoying issue where notification sounds don't trigger around 50% of the times. The notification appears on the lock screen which lights up, but no sound is played. There doesn't seem to be any rhyme or reason to the occurrence of this issue, as it happens to different apps and at random times. Strangely enough I can get a soundless notification from an app, and a minute later another one appears for the same app but with sound this time. I've made sure my phone is not in silent mode, nor in night mode, that my ringer volume is 100% up, and all apps that I expect to have sound notifications have permission to do so. I don't mind so much that I might not immediately catch some mails, but I've already missed some calls and messages in the LINE app (which me and my circles use extensively) this way which is rather inconvenient to say the least.

Restarting the phone doesn't help (nor does a restart using both the power and home buttons), and I'd like to avoid restoring the phone unless absolutely necessary as that would mean losing my chat history in LINE amongst others. So I just wanted to know if there are others out there experiencing the same issue, and whether or not they managed to fix it somehow?