still wouldst thou sing and i have ears in vain
to thy high requiem become a sod
The speaker is expressing his feelings of sadness with some touch of regret because the other person will continue to sing but he could no longer hear it. The song will become a requiem or songs of prayer for the dead and will become a sod or the crust of the ground. In other words, the speaker will be dead or is dying and he feels that the other person's singing will only serve as covering to his corpse that will have been buried under the ground.