pulseaudio-0.9.16_rc4 and pulseaudio-0.9.16_rc4-r51 ebuilds with USE=udev must depend not simply on sys-fs/udev, but on sys-fs/udev[extras]. Explanation: Pulseaudio's module-udev-detect uses SOUND_INITIALIZED udev property to look for sound cards. If no devices have SOUND_INITIALIZED property set, the module will fail to detect any sound cards, and give you only the null output plugin. Udev only sets SOUND_INITIALIZED property if the 78-sound-card.rules rule file is installed. The udev ebuild only installs 78-sound-card.rules if it is built with USE=extras. Therefore, pulseaudio must depend on udev[extras]. Not taking this dependency into account may be a cause of bug #276995 - at least, it caused identical symptoms on my machine.
Thanks for reporting, fixed now.