Takahashi-san ga Kiite Iru.

From MangaHelpers:

Takahashi Ena is a high-school idol with a secret hobby. She often wears headphones, and people assume she is listening to music. In actuality, there is no music playing; she is eavesdropping! She prefers to listen to two of her male classmates, Nara and Mikage, because they have the most interesting conversations. She has to be careful to not be found out, though, because a creepy hobby like that could ruin her image! [tethysdust]