Just got a Pi 3, downloaded the official Pi image from the plex site, wiped the microSD, and loaded the PMP image onto the microSD, and then booted it up in the Pi. It boots, connects to PMS and the video works perfect. However, there is no sound when connecting HDMI -> TV then using built-in TV speakers. This was tried using both latest official 1.1.3 and 1.1.4 RC1 releases, doing a microSD wipe and reflash between each.
As another note, I know the hardware setup works because if the latest RasPlex Pi 2 version is loaded onto my Pi, sound works over HDMI with no problems. Looking at the config.txt contained in the plex images, hdmi_drive=2 already exists. What else can be tried?