Re: gtx400 series audio codec
There's an emulated sound device on all VP4 (GT210/220/230/240/310/320/330/340/GTX480/GTX480) and VP3 (8200/8300/9300/9400) hardware.
VP4 supports dual audio outputs over two digital ports (DVI/HDMI/display port). If you look in device manager of a VP4 based card there should be 4 HDMI audio devices (two per port).
However an output won't show up in audio devices until you connect something (receiver/TV) which can accept audio.
Just like ATI's HD4000/5000 VP4 hardware can do uncompressed upto 7.1 PCM audio (uncompressed/RAW) @24 bit 192 kHz or lossy bitstreamed (undecoded) standard def audio (Dolby Digital/DTS). (The HD2000/HD3000 could only do uncompressed stereo or lossy DD/DTS).
HD5000 can do (bitstream) compressed (somewhat lossy/undecoded) HD audio Dolby Digital Plus/Dolby TruHD (This is based on DVD-Audio's lossless MLP audio) and DTS-HD and DTS-HD Master Audio.
Currently no nvidia card can bitstream lossless audio.