Heard melodies are sweet, but those unheard are sweeter still.

Literal

Audible melodies [topic-は] beautiful but, inaudible melodies [topic-は] even-more beautiful.

A poetic/literary sentence, likely a translation of Keats. Uses contrastive は twice — the heard melodies vs. the unheard ones. さらに (even more, still more) intensifies the comparison. The sentence has a proverbial, aphoristic quality.