The speaker of ‘North’ describes the experience of standing on a beach (“a long strand”) on the Atlantic coast of Ireland (“facing … Iceland [and] Greenland”) and “suddenly” finding himself imagining the Vikings (“those fabulous raiders”) and being inspired by them to write poetry (“lie down in the word-hoard … compose in darkness”).
So in this context the “voices” belong to these imagined Vikings. Their voices were “lifted in violence” when uttering war-cries as they raided the coastal settlements of Ireland in the ninth and tenth centuries. An “epiphany” is a “sudden manifestation of appearance of some divine or superhuman being” (OED), or figuratively, a sudden realization or inspiration. This is a transferred epithet: it is really the speaker who is experiencing a sudden inspiration, not the Vikings.