The CO2 doesn’t help, but it’s not the reason it goes out. A candle can be extinguished by a fan or wind blowing regular air with normal oxygen richness just the same.

So sound is caused by something vibrating. Any sound that's coming from the boosters vibrating air molecules outside which vibrate the windows which vibrate the air inside would fade out to nothing as air outside got thinner. But any sound coming from the boosters vibrating the vessel itself would still be heard even as the vessel got completely out of the atmosphere.