Tunefulness is the noun: the quality of being tuneful — the pleasantness of a melody, the fact that music is nice to listen to. Where tuneful describes a thing and tunefully describes an action, tunefulness names the quality itself, so you can talk about it, praise it, or say a piece of music is short of it. This is a rare word, and it lives almost entirely in writing about music — reviews, essays, programme notes. You would hardly ever say it out loud in conversation. It usually turns up when someone is weighing tunefulness against something else: "the tunefulness of his early songs", "a composer who valued tunefulness over experiment", "what the album lacks in tunefulness it makes up for in energy". That comparing move is really its natural home — a writer uses it when melody is one quality being measured against others. Being an abstract noun, it is uncountable: the tunefulness of the music, not "a tunefulness" or "tunefulnesses". The opposite quality is tunelessness.

旋律優美（這件事）、悅耳動聽的特質。是把「好聽」這個性質變成名詞來討論，樂評文章才用得到，講話幾乎不會出現。中文常常會拆開講：「他早期作品的旋律很美」，不會硬翻成一個名詞。