Change sound level when using earphones with iPhone dock
The short answer is "no". 

Although it shares a connector type with headphones, the 1/8" mini stereo jack on the Apple iPhone dock is a line-level audio output. Its purpose is to connect the iPhone line out (part of the dock connector) to devices like amplified speakers, stereo amplifiers, etc. These devices provide their own volume control and (aside from fine-tuning of input levels) the assumption is that you don't want to have control over the sound level in two places.