2 additional possible options with equipment you may have on hand.
You can always capture the audio on you PC's line in, then use that audio to replace the audio of the iphone video.
If you have a laptop with a good enough built in webcam, and a stereo line input next to the headphone jack, you can record from that camera & use the line in instead of the microphone audio when recording the video.
Do this. I tried it before, and it works well. Here are the steps:
* Plug Pong video-out into TV, audio-out into PC mic-in (or line-in).
* Point camera at TV. Put it on a tripod so it doesn't move. Stand far back and optical-zoom in to fill the screen, and minimize pincushion distortion. Turn off auto-exposure and mess with the manual settings for aperture / ISO until it looks good.
* Start Audacity on PC. Adjust input volume so the peak level is nearly at, but not touching, the top/bottom of the graph.
* Hit Record on Camera and PC.
* Fire up Pong and play some rounds.
* Later, dump the camera data to the PC.
* Open the video in an editor, import the audio, synchronize the two, trim, export, and post!