Understanding Ode on A Grecian Urn

STANZA TWO 

In lines 1-4 I contrast the ideal (in art, love, and nature) and the real - the "heard melodies"; which does my speaker seem to prefer at this point? How can you tell?

The speaker prefers the unheard melodies, the musicality that the urn conveys that can´t be heard but is imagined by him. He seems to prefer what he fantasizes about the urn than what is really there. The speaker uses his imagination to reflect upon the urn and give a more deep and descriptive idea of it instead of sinply staying with the description of the simple urn. The urn is static but what he creates in his mind is dynamic. He gives vividity to the simple urn. I can tell becuase it says "those unheard  Are sweeter", he believes that what you imagine is better thet what you can hear.  At this point the speaker is reflecting upon reality and imagination, he seems also to idolize the urn in some way. 

Is the idea of unheard pipes an oxymoron?

Yes, because the pipes make sound and "unheard" implies silence.This two contradicxtory ideas are contrasted to reinforce the idea of the melodies. There is ambivalnce in this case, between what the speaker can really see and what is in his head. 

In lines 5-10 we begin to sense a negative undercurrent to the ideal, to frozen time. How do I use language to help convey this negativity?

John keats uses the language to convey it negativity. For example, when it says "trees be bare" it is reffering to a tree with no leaves, and the lack of life. This can be a reflection of the speaker upon his own life and how it is empty, his own frustrated life. The repetition of the word never reinforces the negativity and frustration of his love life. Also there is repetition of the word "not". In this part of the poem time is frozen,the speaker doesn´t convey the same idea than in the first lines where there is vividty and a great use of his imagination instead there is stillnessandstatic images. In this part there is a pessimistic point of view of view of life.