Yes they added a tiny stereo ADC which can either be added to existing RGB2HDMI carrier boards (or the RPI) via an add-on PCB you solder on-top (people are starting to make newer carrier boards with it integrated but I don't think there is a Atari shifter one yet).
https://github.com/hoglet67/RGBtoHDMI/wiki/Audio
If you look carefully at the photos previously in this thread from chakobsa you can see the audio add-on board soldered on-top of his Pi.
