Can audio be transmitted over composite cable?
If I understand what you're doing, you want to put the chromecast on the projector on the ceiling, and use two of the RGB to run the audio out back to the receiver.  

Yes, it will work, I have done this many times to re-purpose cables installed in home theater and commercial AV.  The RGB cables for component video (RGB) and composite video (yellow) are just 75 ohm impedance coaxial cables with RCA ends, same type typically used in red and white for stereo audio.  The color coding only matters to the person plugging things in.  

Per google support (https://support.google.com/chromecast/answer/6280276) you'll need a 3.5mm to RCA stereo adapter cable.