I've noticed the speaker volume is quiet. I think they have it throttled back some to keep you from blowing it out. My ringers are super quiet I can hardly hear them most of the time.
This seems or be more of a YouTube thing. It has more to do with the fact that volume levels are different for every YouTube video. My old thunderbolt did this and so does my s5. I have connected both devices to powered computer speakers and on some videos I still couldn't get the volume to an appropriate level.
They had to insulate the speakers, in doing that, it reduced the volume. I either use my JBL flip or ear buds.....I've never enjoyed it when someone used their phone as a jam box.....seriously, a friend came over the other night and played a couple of songs on his android phone...I was in hell.