You have to have a reason to take the towel. Talk to Edna about the towel and muffling.
Edna will only lend it for its intended use: muffling noisy students.
She doesn't hear any noise in the library.
Well, the festival is outside, where all the students are. Making noises. Luckily the noise is blocked by the building.
Open the library window.
Maybe the library door is still blocking the noise.
Jam the library door open so that Edna can hear the sweet student noises.
Talk to Edna. She will give you the towel.