This was made for Samsung Galaxy S3 and tested on it. It should work on other phones, but I can't be sure. I don't held any responsibility for any damage you do on your phone.
The default SMS/MMS 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, a normal sound level for ringer and hear not only incoming calls but also SMS/MMS.
This uses Xposed framework (http://forum.xda-developers.com/show....php?t=1574401)
After you have installed Xposed framework, install this app. After that, enable the mod inside Xposed Installer (you probably have a notification to do it, you can click it). Then reboot your phone (you can soft reboot inside Xposed Installer).
NOTE: This only works in default SMS/MMS app, others apps won't work.
Source code: https://github.com/OXINARF/SMSInRingerStream