I saw this on Pinterest and decided I needed to try it out. I can't believe how much the glass actually amplifies the sound (I don't know whether I'm using the word amplify and speakers correctly here..I know nothing about that stuff.). I kept pulling the ipod (you could use your iphone) out of the glass and putting it back in because it was such a difference. Fun little trick that takes no time or money!
If you have a cover, be sure you unblock the speaker area. I have one of those tough cover things where the bottom is also covered, so I took that cover out (the part where it charges) and that made it even louder.