This is one of the reasons why I love the retro game community so much because long after the lifespan of a console is over people are still creating really amazing hardware to keep systems going.
https://zzblogs.wixsite.com/home/single-post/pnpgc2
UPDATE Apparently if version 2 of this HDMI adapter is moved during use it can brick your GameCube. The creator is moving to make a version 3 that doesn't do this but MetalJesus has an update video about it:
https://youtu.be/wvd-73clV6E

Yeah, you can get stereo and hdmi for your NES these days if you're comfortable with hardware modding. It's amazing how people push the old tech.