You're focusing on a stupid detail.
I can think of dozens of houses I've been in where I could be in an upstairs bathroom with the door closed, and there would be a 0% anyone could walk up or down the stairs without me knowing, even doing so normally.
If they were bouncing up and down and smashing into walls, even more so.
Mitchell, being more intelligent than you, wisely accepted the more reasonable part of the story (hearing them bounce down the stairs) and focused on the much less plausible details (that she somehow knew they were talking, even though she couldn't hear them).