This month Microsoft released new firmware for the Surface Pro 2 and Surface Pro 3 tablet versions. The updated firmware is focused almost entirely on providing better support for Miracast, the wireless video display technology. The update comes with a new video driver which requires an updated UEFI and audio driver to support it.
Each month that Microsoft releases firmware for its Surface tablets, delivery is provided over Windows Update first and then handed off to the Microsoft Download Center a few days later, so Enterprises can build standard PC images with the latest fixes and drivers.
Late yesterday, Microsoft made the May firmware available for download.
You can find it here: Surface software, firmware, and drivers