How can I adjust volume of audio output with iPhone 5s dock?

I just picked up two of the iPhone 5s docks (one for home, one for office) to use with my iPhone 5 since it works fine with it but there does not seem to be any way to actually adjust the volume output from the headphone port on the back of the dock. Given that the main reason I purchased the docks was to be able to plug my headphones in without the phone having to be laying on my desk (or, more importantly, in my lap, in a pocket, etc.), this is kind of a big deal. Is there really no way to adjust the volume output?

The volume buttons do nothing and in the Music app the volume slider is disabled so I can't adjust it there either.