As ridiculous as this sounds, I have spent the last 2 days fiddling around and diving through songs and I finally found a fix that worked for me. I have the galaxy s5 through Verizon...go to Settings>>>Applications>>>Messages>>>Notifications>>>Notification Sound. When it pops up to let you pick what sound you want, turn the volume up or down using the button on the side of your phone. I have no idea what made me think of this, but it worked and I finally have sound on my text message notifications. Hope this helps everyone!!