assuming you have gstreamer tools isntalled, at a terminal prompt:

gst-launch pulsesrc device=source.hw0 ! audioconvert ! volume volume=2.5 ! pulsesink device=sink.hw0

As M.AG says you will experience a delay between audio in and routing out but depending on the applicaiton you want to put it to this may not be a killer for you.

I've added a dash of volume as I find the output a little quiet otherwise. Feel free to change the value to taste.