Final answer:
The lion's roar represents an encroaching realism and danger within the Hadley's technologically-created virtual world, symbolizing fear and a loss of control as the lines between virtual and reality blur.
Step-by-step explanation:
The lion's roar in the context of the excerpts you've provided seems to symbolize an element of realism and danger that is becoming all too tangible within the Hadleys' lives. As the technological marvel of the nursery brings fictional and fantastical elements to life, like the African veldt, it becomes increasingly difficult for the Hadleys to distinguish between reality and artificial creation, especially when the lions' presence is felt through their roars, the smell of their fur, and even the visual of the lions themselves looking at George and Lydia with 'terrible green-yellow eyes.'
The reactions of the characters—Lydia's nervousness and George's contemplation—demonstrate how the roar and the lions are affecting them, implying that this advanced technology, while initially a source of entertainment, might now represent a threat or challenge that needs to be addressed.
When George finds an old wallet that had been in the lion's mouth, wet and with tooth marks, it is a strong indication that the lines between the virtual and the real are blurring. This illustrates that the lion's roar, beyond its presence within the virtual nursery, signifies an encroaching fear and a loss of control—themes that are core to the narrative being described.