This is one of the things we face on media over the internet (even with computers). Taking Utube as an example, there is no, or, very loose standards for recording levels in place. So, different videos, will vary in both sound level and quality.
Some things you can look at, is on the video site is, if they offer a volume control for the media being played. And, the phone's speaker system isn't the best for multimedia, you might want to try a good set of ear buds.
IM