In what way are the scenes depicted on the urn superior to their real-life counterparts?
The speaker views the scenes depicted on the urn as superior to real life in two ways. Firstly, the scenes are beautiful. They present an idealized look into daily life in the ancient world. In a way, they are more beautiful than real life because of the medium in which they are presented: still images. As the speaker observes when looking at the musician on the urn,
Heard melodies are sweet, but those unheard
The spectator can never hear what the musician is playing—he is merely an image. But the music the speaker imagines is likely better than anything the original artist could have come up with.
Secondly, the images on the urn will remain beautiful as long as the urn exists (which the speaker appears to assume will be forever). The young lovers will never grow old. The tree's leaves will never be shed with the coming of fall. Death, too, will remain a stranger to the figures on the urn (an irony, considering that the purpose of an urn is to hold a person's ashes). All will remain as lovely and young as they were the day their creator made them. Unlike in the real world, where time's passing has consequences, art remains unchanged.