Since you have picture and not sound, you probably already have a box. Check the connections. How do you have it connected to your TV? If it is connected through coax, make sure your TV is on the correct channel.
Also, try swapping the box to another TV to see if the problem follows. If the problem follows either you are connecting it wrong on both TVs, or the box needs to be replaced.
Make sure on the back of the box that you are going from the audio output and not from a different video output. There are red, green, and blue video outputs on some of those boxes and if you try connecting a cord between the red video output into the audio input of the TV, you will usually get a buzzing noise.
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