Some of the sounds are low. I had to pick one that sounded louder to me. You could also download and use zedge and pick a notification sound from there. Sometimes they have some loud ones on there.
Wait. Is your notification volume all the way up? Your regular ringtone volume is probably all the way up. You have to go into settings>sound to check the volume levels on your notifications, media, system and ringtone.
The volume levels for your notfications, ringtones, and system are set separately under Settings/Sounds. I noticed this same "problem" until I went back into settings and reset the notifications volume.