The sound of a voice that is still. (12)
Again, the speaker longs to speak with his dead friend, but he doesn't imagine chatting with him – he only imagines a disembodied "voice" that is now "still." Instead of imagining his friend in his entirety, he imagines him only as a series of absences.
A day that is dead (15)
This is the only time the word "dead" comes up in the poem, so it must be important. But the speaker doesn't describe his friend as "dead" (only as "vanish'd" and "still") – it's the time that he spent with the friend that is "dead" and gone.