This may sound like a stupid question, but have you removed the protective plastic from both sides of your iPhone? You'd be surprised at how many folks forget to do this. If you already have, at least we can go from there.
It is surprising how many times the problem you describe is for the reason Napoleon mentioned above. If that was your problem, don't feel bad, you are not alone. Aside from that, the volume settings for the built in ringer and earpiece are adjustable using the volume buttons on the left side of the phone, near the top. The uppermost button raises the volume and the lower one lowers the volume. The volume of the built in earpiece is generally quite good and on some calls I need to lower it somewhat. The ringer volume is another story however and is the only complaint I have ever had with all of my iPhones I have owned. Re-recording the ringer tones used for the ringer, at a higher volume, has helped but is not the answer. The audio power to the speaker, on the bottom of the phone, is simply not enough. (are you listening Apple?) If I am wearing a coat, I won't hear the phone; if my wife has the phone buried in her purse, she won't hear it. I had a Motorola Q, before I got into the iPhones, that would practically wake the dead when it rang.
I just saw your answer to Nap's attempt to answer your question when I posted the above and refreshed the page. Calling someone's answer stupid or the person themself a dummy is not a good way to start out as a new member on this or any other forum. Your turn Naps.
Because some people are morons. I thought it was a legitimate question to ask. Many don't want to take the plastic off the iPhone right away because they're afraid it'll scratch before they can get a case on it.
Oops. I wasn't ready to send that last reply. Secondly, if a reset all settings or software upgrade does not fix it I would think it is perhaps a hardware issue. If you are in an area with an Apple Store close, you might try an appointment with the genius. I find that the 5 has the best and loudest sound of any iPhone so far.
I would say something about wax removal from the ears, but that would be rude.
Check settings, sounds to make sure the slider is all the way to the right & during a call click the volume button (top one) to make sure it's turned up also
Try a hard reset. If that doesn't work, reset all settings. If the audio problem is still there after that, try a restore to factory settings. If you're experiencing it at that point, it might be a hardware problem. It's still under warranty and Apple will exchange it for you.