.. Eavesdropping Edna in the interview room

So, you would need to hear through the door? I think you need some item to help you with that.

Did you read the books in your office? ( Especially the I Hear You: An Audio Primer for Curious Cats )

In the book called I Hear You: An Audio Primer for Curious Cats they say you can hear better when you press something hollow against a surface.

Like, a glass, for example.

Did you do any investigation around Edgar's body?

Examine the stain left from Edgar's body. The glass is on the floor. You can pick it up.

Use the glass on the interview room door to hear what Edna is talking about.

...the cake, yes. Maybe a new cake would make her feel better.