How do I turn off speech search in Jelly Bean?
I do not know of a way to disable Google Now intercepting media button presess unless you have a rooted phone, as you did not specify that you did, I am going to assume you don't.

The easiest way to do this would be to disable the Google Voice search app.

Go to your Apps settings menu (System Settings --> Apps).

Slide to the left until you get to the 'All' list, scroll down until you find the app named 'Google Voice Search', click it and click the disable button - now reboot your device.

Edit:

As you have a rooted phone, you could install the Auto starts app from Google play, and use it to disable the 'Media Button Pressed' entry under Google Voice Search.

This is a paid app.

Google Voice Search will not act as if it is not installed onto your device.