Can't get 5.1 from RTX 3080 to LG OLED B6 via HDMI, only stereo. Philips HTS3578W system.

Hey folks, I'm having a weird setup issue and hoping someone has run into this.

I have a Philips HTS3578W DVD home theater system. My current connection is:

• **PC (RTX 3080)**​ → HDMI​ → LG OLED B6 TV
• TV​ → Optical/TOSLINK​ → Philips HTS3578W

The goal is to get 5.1 surround sound from my PC to the Philips system. The problem is, my **LG B6 TV only reports "Stereo" as an audio output option from the PC over HDMI.**​ In Windows Sound settings (Playback devices -> LG OLED B6 -> Properties -> Advanced), the only format available is 2-channel stereo. There are no options for Dolby Digital, DTS, or 5.1.

What I've tried (and failed):

1. Setting Windows audio output to the LG TV device and checking all "Advanced" format options.
2. Using the "Configure Speakers" wizard (only lets me select stereo).
3. Updating/clean installing NVIDIA drivers (including HD Audio driver).
4. Trying different HDMI ports on the TV, including the ARC port.
5. **Even tried using CRU (Custom Resolution Utility)**​ to manually add Dolby Digital/DTS support to the monitor's EDID. No luck—the options still don't show up in Windows.

It seems like the B6 is telling my 3080 "I only accept stereo PCM over HDMI," and that's all Windows will send. I've confirmed the Philips system and optical cable work fine with 5.1 from other sources (like a Blu-ray player connected to the TV).

**Has anyone else with an older LG OLED (like the B6/C6/E6 series) and a modern GPU managed to get 5.1 audio passthrough working via HDMI -> TV -> Optical?**​ Is this a known limitation of the TV's HDMI input board?

Any workarounds or ideas would be hugely appreciated. Is my only real solution to use an HDMI audio extractor​ to pull the audio directly from the GPU before it hits the TV?

Thanks in advance.