If this is a brand new ATV, what audio are you getting? There's a bit of setup to do before you can play anything, and you need the menu to do it.
The brand new ones just plug into HDMI and work. If this is a first gen unit that plays the startup sound, you could have a broken unit. All 1st gen ATVs would be used at this point, so it could be set to some video settings your TV doesn't like. You didn't specify the make/model of your TV, so it's hard to say. HDMI is supposed to work that out, but there are settings on both the TV and ATV that could lock either one into some incompatible state.