I have an hp dv9347cl laptop with a Geforce Go 7600. HDMI audio out works in windows using an older nvidia driver. It does not work in linux using the nouveau driver. I also have a Geforce 9400GT card in my desktop with the SPDIF jumpers, connected to my MB, that works great in Linux using the Nouveau. This tells me that there is at least the potential for HDMI Audio pass-thru to work correctly on my laptop.
There is only HDMI audio support starting with NV84. If you're interested in helping to get this to work, please collect a mmiotrace of the nvidia proprietary driver that captures a plug and unplug HDMI event. (i.e. start it without hdmi, then plug hdmi in, then unplug it.) Please be sure to add trace markers (echo stuff > /sys/kernel/debug/tracing/trace_marker) before/after you perform each of those things. I guess it wouldn't hurt to also try playing something over HDMI, but I doubt that anything actually happens when you do that. [Please make sure it's an audio-only thing, as video would rather complicate the trace.]
There are some instructions on how to do an mmiotrace at https://wiki.ubuntu.com/X/MMIOTracing
The resulting trace file should be 50-100MB, so please compress it before uploading. (And if it's just a few KB, you did something wrong.)
Also upload a copy of your vbios (/sys/kernel/debug/dri/0/vbios.rom if you're running nouveau).
I'm not sure if HDMI audio support was added for NV4B, but it might be worth trying a recent kernel (3.18, 3.19-rc4).
If it still doesn't work, please provide the traces Ilia asked for.