They make the following kinds of adapters. I only have the first one.
I would hope the computer only routes sound to video connectors that
are in usage. If your video card had three connectors, it's not necessarily
the case that all three have the same audio signal at the same time. But
I haven't tested this. That would require me to buy more of these.
HDMI --- (box) ------- VGA
------- Stereo Audio
------- (some do SPDIF, but that's a different product)
DP --- (box) ------- VGA
------- Stereo Audio
These can also be equipped with a miniUSB
solely for +5V power. Even though the video card
DP and video card HDMI have 5V power pins on the
connectors. My video card provides enough power, that
any of the adapters I've tried, they worked.
Some equipment in your AV rack, could have poor +5V on one
of those display connectors, and that's why the adapter has
an additional (optional) +5V input.
Things like soundbars can do audio-drop to run their speakers,
then pass an HDMI-out to a TV set. This allows the soundbar to
have audio and the TV to have video.
Generally, I don't recommend buying roomfuls of toys of
this nature. Test the kit you bought first, and see if it is
satisfactory. Maybe you have enough goods right now to get
the job done. Maybe the motherboard audio is "good enough".
Adapters could range from $30 to several hundred (for Apple docks)
that do conversions. Some of the adapters could be more expensive
than the video card they're connected to.
Sound cards vary a bit on background noise level. A Creative
with a shield box around it, might be -108db. Computer sound (HDAudio)
could be -70db. The worst part of computer sound, is when you move the
mouse and "digital noises" can be heard in your speakers. Concert hall
music can have sufficient dynamic range, to need a clean noise floor.
Many other kinds of music don't really need to be all that clean.
But things like mouse-noises, those suck. Sometimes you get stuff
like that from a grounding problem in the computer room.
These are moments when you start retrofitting stuff.
I haven't seen any HDMI to four 1/8" connectors for 7.1 sound. HDMI
and DP should at a minimum support 8 channel LPCM (linear pulse code
modulation), which is the "patent-free" audio format on video. There
are various Dolby options no one cares about. A sound bar might pick
off 5.1 and drive its own Sub (as your DVDs might have a 5.1 audio track).
Your new motherboard might also have TOSLink, which is SPDIF sent
via red LED light. Which is fine if you have an AV receiver with
a TOSLink input. Otherwise (like for me), the TOSLink is a waste
of time. My amp is one I built myself and there's no TOSLink on it :-)
You could send audio over Bluetooth... but... don't do that.
It's pretty awful. It's like listening to the lowest bit rate MP3.
There is a software product, RightMark Audio, that can be used to
test computer sound. I'm not recommending a copy, just look
for a reviewer that uses it and see what it can show. SO if you
were interested in some Creative soundcard, you could combine the
search term "RightMark" and get some graphs.
Paul