Recently I got some issues with Hangout not waking up or making sound when receiving new SMS.
After some research it seems to be related to playstore issue.
Got a brand new stock Nexus 5, rooted it, installed bunch of app and then no more notification for SMS in hangout.
Did a wipe, did not install anything, Hangout was working fine.
Installed patched playstore, Hangout and some Google Now functions stopped working (like automatic location update, notifications, etc...)
If you installed a modded playstore, revert back to stock or wipe to fix it.
This is particularly true with LuckyPatcher cracked playstore that broke links between Playstore and other google apps.
If you dont want to wipe or keep your modded playstore, use GoSMS pro for notifications.
There is an issue with official Playstore that drain battery because of kernel partial wakelock for nlplocator. Using modded playstore broke the automatic nlplocator and this way fixes the batery drain, but you lose functionnality especially about location informations (google now will only update your location on user interaction).
Hope it can help talented person that wants to investigate this issue.