Annonymous Noise anime

Every day, a young girl wearing a mask stands by the beach and
sings a nostalgic melody. After experiencing two sudden
heart-wrenching partings when she was only a child, Nino
Arisugawa has been singing her songs to the ocean, bound by a
promise made with her two childhood friends—her first love, Momo
Sakaki, and a boy who composed music, Kanade "Yuzu" Yuzuriha.
Having never met each other, the boys both individually promised
that if Nino was ever separated from them, her voice would be the
beacon to reunite them once again.

After six long years, destiny has finally placed Nino, Momo, and
Yuzu in the same high school. However, the passage of time has
changed many things in their lives—while Nino relentlessly
attempts to fulfill her childhood promise with the boys, Yuzu's
feelings for her from the past resurface, and Momo goes to great
lengths to prevent a reunion with Nino. Through music, will they
be able to mend their friendship and overcome all the feelings
involved in this complicated love triangle?