I have the V-Moda Crossfade LP headphones, which have convenient volume buttons. They work great on my iPhone, but on my Nexus 7 they seem to have no effect. They even work on Windows XP, 7, and Mac. How can I make them work on Android too?
Unfortunately, the answer is that you can't make it work. (at least, not without modding your Nexus 7)
As jeff says in his comment:
I can confirm unlike some other android devices I have used standard volume control headphones that work with iPhones and many other mobile devices, including android do not work on the Nexus 7. This is a hardware issue as far as I can tell
According to the Crossfade LP product page, the volume controls are only for apple devices:
Universal Compatibility and Microphone Communication LP comes with 2 cables for universal compatibility with all modern mobile and audio devices. The 3-Button remote microphone cable is designed for the latest Apple devices, including iPhone®,iPad®, iPod®, and Macbook series. The long audio-only cable and 1/4" adapter is universally compatible with all audio devices and professional equipment. (Emphasis mine)
In order to understand how these headphone volume controls work, I asked about this over on Electrical Engineering stack exchange, and it appears that this relies on the switches presenting a fixed resistance between the ground and the microphone input. Thus this technique requires (at a minimum) a 4 pole (TRRS) jack, rather than a standard 3 pole (TRS) jack as is used in the Nexus 7.
Note while that the Asus Nexus 7 spec suggests that it has a Headset Jack (i.e. headphone and microphone), Google have confirmed to at least one person that this is incorrect, and the Nexus 7 only has a headphone jack.
As such, the volume controls are not intended to work with ordinary headphone sockets such as the one on the Nexus 7.
I'm not sure how you got it to work with Windows XP/7 unless the headphone socket on your windows machine was also a headset jack and also supported this volume control technology (or you are running them virtualised on a Macbook, in which case OS-X will be taking care of it for you).