GET A CAN OF COMPRESSED AIR!
MY STORY: I accidentally placed my iPhone on an oily kitchen counter. No one could hear me unless I used the earpiece or the speakerphone (which was creating an echo effect on the other person's end).
I went to Apple at the advice of a Sprint store associate who felt confident that Apple would simply give me a new phone, since I'd purchased it a month ago with warranty. I went to 2 Apple stores & was highly annoyed by the whole experience. Even though the liquid indicator hadn't been triggered (not sure why), they all said that my warranty was now voided. A "Genius Bar" staff member took the phone apart & showed me a light oil film on the battery pack. I asked about cleaning it out myself & he said I couldn't do that, nor could I be allowed to open it myself because it would void the warranty (But isn't it already voided from the oil?!?!). It was suggested that I buy a new one, which wasn't an option on my radar.
I wanted to go home & take it apart to clean it myself, but Apple has different proprietary screws for each iPhone model so it was impossible to find. When I asked a nearby Radio Shack for a pentalobe screwdriver, they immediately responded that they don't carry tools for iPhone 4 (I must not have been the 1st person to ask). Instead of ordering online, I went ahead and tried a can of compressed air. In seconds, VOILA! It's back to normal.
AFTERTHOUGHTS: Now, I'm a little bitter towards Apple b/c the ppl they put in place to "help" and "repair" weren't allowed and/or trained to offer me any practical assistance, only a discount on a new $450 phone that I had just purchased. They were just sales-wolves in sheep's clothing. Even when I accepted my fate w/the warranty, I wasn't allowed to try my own methods of repair b/c Apple has gone out of their way to keep me from being able to open the phone.
At the very least, I could've been permitted to take the dissembled phone home w/me, since it's all void to them anyway. But no, they "must" reassemble it. Maybe I should've smashed it & lied since the liquid indicator never went off. IMHO, honesty is the best policy most of the time, but not all of the time. If you simply can't afford a new phone right now, I think you should do what's best for you.
In sum, TRY SPRAYING A CAN OF COMPRESSED AIR INTO THE OPENINGS ON THE PHONE. It worked for me. Good Luck!