I spilled a drop of balsamic vinegar on my iPhone 4S near the earpiece.
Now, call volume on the internal speaker is very low. How can I clean the earpiece out?
You will have to open your iPhone, e.g. as described on iFixit. Then you can clean the affected area with a very gentle cleaning solution (but be aware to not use any solutions that may harm your hardware).
And for the next time: Don't place your cell phone near your salad. Balsamic vinegar is an aggresive solution which may severly damage pieces of your iPhone.