How does the built-in Audio of the Thunderbolt Display compare to the MacBook Pro Retna?
According to the technical specifications, the thunderbolt display has 2.1 stereo speakers with a subwoofer and the system is rated at 49 W.

No one would consider the any MacBook Pro with the small internal speakers to be superior to the wall-powered solution that is part of the display.

You will want something like an external DAC if you are concerned about audio quality or want to connect speakers that are better amplified and suited to a special situation other than casual listening. The Display Port monitor is better than many $150 speaker systems, but you can also get quite amazing external sound starting at that price point in the US.