Alright, so I measured it. What a pain.
Anyway, my TV is a LG OLED55C9 that was fed by a Windows PC using HDMI and measured via RME ADI-2 Pro FS using REW 5.20 on a laptop. No ground loops were present.
The results were about what I suspected. Not too good but not too critical in practice either, although it does just about everything wrong that it can do wrong.
We have clipping at full volume, rather high noise floor and high output impedance.
Measurements without a load:
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At full volume there is heavy clipping. At 75 volume (Windows volume control) there is no clipping but the SNR is limited to 73dB. The performance is best at about 55 volume with a SINAD of 79dB. Weirdly there is a power supply spike at 60Hz when the mains frequency here in Europe is 50Hz. Never seen that before.
Under load that output voltage collapses:
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The calculated output impedance is a whopping 149 ohms wich is very high. This output would work better with high impedance headphones. 50mV SINAD with 33R load is 67dB wich is rather poor as well and might be audibly problematic.
All in all not a very good showing for a 1500€ TV. But then again, not many people would use the internal DAC/amp so it's probably just fed from the amp for the internal speakers through a resistor.