My hypothesis on this is influenced by this article from National Geographic and this article, along with my status as a cognitive science healthcare provider.

Verified in rats and likely similar in humans, low gamma waves connecting the brain for searching memory and our sensory areas for vision and hearing may become hypersensitive because it is aware of a crisis (death). This causes sensations of light, ability to hear conversations while dead, and since the ears are involved with balance, the floating sensation.