What kind of cable/connector? It could be stereo (left, right, ground).
I assume the Arduino is putting-out mono. In that case, you only need two conductors. If the amplifier is stereo and you're connecting two speakers, connect the left & right inputs together.* You can do that at the Arduino-end (connecting the red & white conductors together) or at the amplifier-end.
...Usually "Aux" inputs/outputs on a TV or stereo system use [u]RCA connections[/u] with a separate connector/cable for left & right.
[u]This is a 3.5mm stereo connector[/u] (3-conductor TRS plug).
Now, i bought a new adafruit chip to amplify the signal
I'm guessing you bought and amplifier board with connectors and some other components, not just a bare "chip"?
- It's OK to connect the left & right inputs together, but you should NEVER "short" left & right outputs together.