If you full-screen a camera that
Blue Iris thinks has audio, you should find this speaker button up in the top bar. Use Firefox for best audio performance.
You'll also see that icon when viewing a camera group, if you have audio enabled for the group, though I am not sure what the audio source is in that case.
Two way audio is not possible right now. Maybe in the future, but I have never worked with two-way audio in a browser before and I imagine whatever the browser is capable of is not compatible with what Blue Iris is capable of.