You might want to try a different notification sound. The sounds are not all the same "loudness." Some are barely audible even when the volume is up high, while others will tear your head off even at a couple of clicks of volume.
I don't know. I'm using the "lime" sound and I feel that might be the loudest. Still can't hear it. I really wish the other text apps looked and worked properly on this device because the options are much better with those apps.
Likewise--too loud here, even on low. Are you sure you've got the separate Notification volume up from the standard volume? The gear icon when changing the volume brings up the sub-menu with the different sliders for the different volume types