I'm gonna quote directly from the source and include the link.
This little trick actually does improve the listening experience - try it. Just put the eternity in upside down so the speaker is pointing down.
/quote/Use a Glass as a Smartphone Sound Booster
Nic cranks it up to 11 with this tip for speaker-less situations:
I rarely use my iPhone to blast music but when me and a group of my buddies were all listening to a new song, one of them (who also has an iPhone) grabbed my phone and a cup... it was amazing the difference and I just sat there for a good ten seconds straight blankly staring at him trying to figure out why I never thought of this before. The iPhone's speaker is down facing and the cup adds a lot of bass and volume—especially when set on a hard surface (like in the shower, you can hear it over the water!).
here's the link - http://lifehacker.com/5443923/from-the-tips-box-smartphone-speakers-wallet-holes-and-fire-starting/