Please explain clearly what audio interface you have and how many ins/out does it have.
The easiest way to convert Reaper in a karaoke app with Youtube video is to route the default windows sound device to one of your interface inputs. Some interfaces let you do this by software, if your interface don't and it have more that two outs then you could simply patch the outs to an input.
Later create a reaper project with one track per mic and one track for the youtube sound that should be set to the input you patched in your audio interface.
Example for an audio interface with 4 mono outs and 4 input channels:
- Set Windows default audio device to outs 3 and 4
- Connect a cable from out 3 to your interface input 1
- Connect a cable from out 4 to your interface input 2
- Create a Reaper project and create one track for each of your mics. Add all the FX you want. Try to add only effects with no latency.
- Create one last track and set the stereo inputs to your interface inputs 1/2. This will be your youtube video sound. Add FXs to this track, I recommend a compressor, some EQ and a limiter
Now just open youtube and play some karaoke. The sound will be routed to Reaper and processed by it.
Hope this helps. I used this a lot with my wife's karaoke parties with excellent results.