Mitsubishi WD73734 issue with HDMI in ---> Digital audio out

The HDMI specifications only require the TVs support PCM Stereo. All other audio is optional. The majority of TVs only support PCM Stereo via HDMI, this is the normal. Later TVs have started supporting Dolby Digital but usually only upto 5.1 and not Dolby Digital HD or DTS Master HD (the TVs cannot use 7.1 and the optical and coaxial outputs cannot handle 7.1 so there would be no reason for support).

The source device - Blu-ray player, Cable Box, Satellite receiver etc.- are required to support the conversion of audio to PCM Stereo - it is HDMI rules.

Your TV, per what you posted from the Owner's Guide, will only accept PCM Stereo via HDMI. It will support Dolby Digital (up to 5.1) from the Antenna input, it has too because Dobly Digital is the audio type on over the air broadcast. The digital audio output of the TV will send the type of digital audio the TV is receiveing (it will not convert digital audio in the TV, just passed thru). When you are receiving analog audio (analog channels or from separate Left and Right inputs) the TV will convert that to PCM Stereo for the digital audio output.