There’s a fairly exhaustive discussion here:
but from the standpoint of what you’re after this is probably a little better context:
…that post covers setting up NDI output from Flame and then grabbing that output with Ultragrid and pushing it to a broadcast monitor. Initially this was so that someone without a router could use Louper as well as have a broadcast monitor, but it still applies here. The only thing missing in that setup would be setting an audio preview destination which you can select from the Audio playback menu.
Hope that helps.