Try change the discord streaming setting:
If the streaming setting was already turned on in discord, you could try to disable it and see if that solves the issue,
the feature is experimental and might not work with all applications, this could be why it wasn't working.
OBS + Voicemeeter workaround:
I've streamed audio though discord using software such as Voicemeeter previously, you could try to combine this with OBS streaming to stream both audio and video.
The goal with software such as voicemeeter is to combine both specific sounds sources (such as vintage story) with your microphone into one audio channel that you can use as audio input to discord.
Here's how I setup this, you enable/disable A/B channels to forward it to the desired output:
You can find App volume and device preferences by putting this in run (WIN+R) -> explorer.exe ms-settings:apps-volume