Space inside the disc was roughly estimated by examining pictures and video, and similar projects.
http://www.instructables.com/id/Custom-Tron-Disc-Mod/?ALLSTEPS
https://www.flickr.com/photos/blackfeathers/sets/72157625208305212/

LED: The RPI can drive WS2812b LEDs with DMA https://github.com/jgarff/rpi_ws281x
This conflicts with PWM sound, but we have options:

• No Sound / simple beeper
• Disable LED updates while playing sound
  
• Use existing disc chip sounds
• square wave sound
  
• I2S audio
• "Upsample" audio to the shared PWM frequency?
• Add a microcontroller for LED strip
  ESP8266: switch between sdio firmware & on board flash
  (using WiFi would disable LEDs)
  
• SPI LED drive - previously dismissed because no DMA
  • But now Kernel 4.0? has DMA support for SPI? spi-bcm2835
    https://github.com/raspberrypi/linux/pull/1085
    for transfer length >= 96 bytes, and < 65536 bytes
  • Aux SPI can do variable bit length (which may help?) but no DMA?

WiFi: The RPI has support for a 2nd SD / SDIO port, and the ESP8266 has a SDIO bootloader mode. https://hackaday.io/project/8678-rpi-wifi
The just released Pi Zero W (if/when you can get one) may make this redundant. But the ESP8266 may have future potential here as a scriptable WiFi/SPI/GPIO expander.

Battery: The "Custom-Tron-Disc-Mod" project above stated AA would not fit but AAA would.
But for this project, with more LEDs -- a LiPo can have a larger capacity (for more runtime and brighter lights), and fit more compactly (allowing more peripherals).
Early plans were to fit the battery in the battery area - possibly removing the screw for the USB jack.
But latest plans are to put the battery in the largest open area - to eliminate sharp corners that could puncture the LiPo.

Other technical hurdles:

• Organizing notes & plans: OneNote App has issues: slow to paste, flaky search, can totally hang up if network issues
• RPI USB Ethernet Gadget trouble (device recognized, but no data transfer)
  • Has worked in the past
  • Always seemed flaky (maybe due to Win10 drivers and some OS corruption from multiple small disk failures)
  • Then worked on one machine but not another
  • Now not working anywhere: may be corrupt SD? (dmesg does show some errors)