So I made a sound card. A USB one. Based on the TI PCM2904 USB DAC. Works a treat, both output and input. No drivers needed, appears as "USB Audio Codec" in windows and Linux. Will drive a pair of headphones at nanny state level, but put it into a decent headphone amp and you will find it is not short of gain.

All SMD with the exception of the white power LED. Air soldered with an 898 clone. All components ordered from RS Components. It wasn't exactly cheap though. BOM and boards for 2 + left overs was about £100.
Designed in KiCad with lots of help around getting the output filter designed from the kind folks on here. Ended up using a butterworth filter from a TI Application/Design note/Demo board.
Full sources for KiCad is available here:
https://gitlab.com/paulcam/PCM2904_DACBoard manufactured by AllPCB for $22 including shipping which only took 6 days end to end. I ordered 5, they sent 10. The boards would have been $15 but I wanted blue solder mask which added $7.
(On the topic of gain, if you want to drive headphones (at all) or an headphone with a bit of kick, swap R8 and R9 for 3.9K instead of 12K.)
It's mind boggling to think I went from asking dumb questions about Ohm's law in October to making a full USB sound card device this week!
Oh and.. jerryk, you have started a trend with your "Harmful if swallowed."

"For external use only!"