Which version of the 37PF9731 firmware have you installed?
I just upgraded to version 101. After the upgrade I lost sound from my Philips-DVD-Player (HDMI) and my Premiere-HD-Receiver (HDMI). Additionaly I had no picture on channels with content protection (Premiere HD). I contacted Philips support, but finally I found the previous firmware (0.96). After a downgrade all was back to normal.