In general...
the responder is someone who is responding/writing back/criticizing on the poem
the audience is us yes, because we are reading the poem as we are the audience if we watch a play or movie but the poet normally intends his poetry for a specific audience eg, yeats' was for irish nationalists and his girlfriend in the early 20th century. this is where context comes in and who would actually enjoy or read the poets work.
but we are definitely the viewer. the viewer or reader is anyone who reads the poem