I took the Xposed Mod originally made by @OXINARF (http://forum.xda-developers.com/show....php?t=2330473) and changed it to work with the Hangouts app, because many of us prefer to use Hangouts as default SMS/MMS app. All credit goes to him.
The default Hangouts app has no option to set volume level for notifications and it uses the level set by the system to all notifications. I prefer to have it use the same level as the ringer so I made this mod to do it. This way I can have a low sound level for notifications, and a high sound level for ringer and hear not only incoming calls but also Hangouts messages.
You need Xposed framework for it to work. Instructions are here: http://forum.xda-developers.com/show....php?t=1574401
Just install the apk, enable the mod in the framework and reboot your device.
This should work on all devices running Android 4.0+.
This only works in default Hangouts app, other apps won't work.