I wonder whether there's also an idea of "hollowness" at the bottom of it, as well.
The shock and disillusion make her feel suddenly hollow, like the shell of a building; when she speaks, her voice sounds like an echo in an empty building.
Or perhaps the shock brings the blood to her head: sometimes when that happens, your own voice sounds distance and echoey.
But maybe that's all too fanciful. I expect it's as Julie says.
"I cheated on daddy."
"You cheated on daddy?"
MrP