WebCord is an “alternative client” for Discord, although it’s just running the Discord webpage in electron. Recently it updated its electron version so it supports sharing audio as well as video.
I tried it out today on mint (x11, pulseaudio) and it works flawlessly.
So you can share audio while sharing your screen just like on Windows. Before the only way was to route it through your mic which is annoying and has a ton of drawbacks.