The first thing you are going to note is the difficulty of hearing it if you are not using headphones. You should always try to use headphones when watching videos on your iPhone, but you can get away with just watching them cleanly on occasion. The first thing is you have to be in an area that has little to no competing sound. This is just going to clutter up the audio landscape and make it next to impossible to hear your iPhone.
The iPhone has surprisingly powerful on board speakers for a cell phone, but they are not that powerful. If you are having trouble hearing them you may want to cup your hands around the back, creating a dome that shoots the sound forward. This will often make it much easier to hear.