The audio data from your network adapter is sent to whatever audio device you select in Windows (click on the speaker in the lower right in Win10, right click on the speaker in Win11 and select Sound Settings).
For example, if you have an external USB DAC and select that device for audio output, then all the data is sent to the DAC, bypassing any other audio device that’s installed.
I’ve never used a USB DAC through a powered hub, so I can’t speak to how reliable it would be. I don’t think it would be a problem if you plugged, say, a mouse, keyboard, and DAC into the same hub. The safer option would probably be getting the expansion card.
If you still prefer a soundcard, I think you’ll have a tough time finding one with RCA line output in half-height form factor.