No audio through displayport

[MR ZenWiz]

I have an HP 8740w "elitebook" laptop that comes with a displayport
instead of an HDMI port.

I have tried every possible combination of DP->HDMI adapter and nvidia
vs nouveau drivers and none of these combinations produces an audio
stream that goes through the digital interface the DP (and HDMI) are
supposed to provide.

All I see in the volume control audio settings are five analog stereo
settings and an off setting.  None of these produce sound through the
DP->HDMI cable, and one of them shuts off the sound through the
headset plug, though it does brighten the sound through the built-in
speakers.

I have complained to nvidia, and their response was essentially, well,
that's an old viddeo card, tough luck.

I did a search for "laptop hdmi plug-in video controller" and nothing
turned up in the way of cards, just troubleshooting tips, mostly for
Windows, which I have already seen.

Is there such a thing at all and who sells them?

Is there another option and if so what?

I really like this laptop, but I'll sell it and get another if this
isn't possible.  Too much of a shortfall.

Thanks.
MR