Hmm... be careful what you say and do not make assumptions.
I've been in the firmware/software/hardware business for over 40 years now. I've been doing R&D since Carter was POTUS.
Heck, it's people like me who created the "firmware" and Internet concepts. When I started we had the 6502 and Z80... now when I can remote into the lab and do things that we would have never thought about just 20 years ago.
Have you even been in a chip design/production chain?
Do you know RTL, real time firmware, JTAG, emulators, simulators, FPGAs, etc, etc...
Can you use a signal/function generator with a scope and program a computer to coordinate measurments.
Have you done internetworking, embedded software, video streaming?
Have you done networked video distribution?
Have you done digital video?
Etc, etc... things that I can not tell you because of NDAs.
So, do NOT assume... do not insult my skill set because it sounds like I can teach you a lot of stuff you don't understand much.
Processing audio separate from video is precisely what is being done in those chips.... you see... normally you take the digital stream data and split the video and audio into different chains while maintaining a master clock. In the better products, you will use a set of ARM cores, a bunch of AXI busses, some kind of a matrix switch, a media processor block, etc...
Indeed, the most advanced SoC's today are in a cell phone. They are expensive but they benefit from huge economies of scale.
What you call an "AV processing chip" I call an SoC ( System On a Chip). That is the standard naming convention in the industry. Nowadays, those SoC are pushing forward with including FPGAs as well.
So you see, the "chip" I want would not be difficult nor expensive to design and build... we'd take a Xylinx or Intel SoC device with an ARM A7 and FPGA.... simple, cheap. Audio decoding is easy.
So, now, what else can I teach you about the TECHNOLOGY and the Physics of it?
The only current reason why we don't have the product is simple... it's MARKETING... because the marketing guys in the consumer world want you to blow money every three years as they keep changing standards. I know this because in R&D we're constantly fighting Sales and Marketing over everything... Bill of Materials, Features, Schedules, etc, etc...
That plus the constant licensing fees that keep getting generated... why do you think Oppo got out of the business? They couldn't make money paying for all those logos on the front panel! Most of which the consumer will NEVER use!
There is no reason why the product I just described can not exist from a technical view point. I just described to you the development platform... you can put it together with available development single board computers (SBCs).
And it would sell... priced at around 1000 bucks there would be plenty of budget to pay for the legal fees, to include high quality components -balanced outputs too, and it would make life very easy for installers and people setting up home theaters ( specially professional installers and advanced hobbyists).
It's just the marketing guys not wanting to give it to you... and people like you, who think they know it all.