It is self-evident, unless one wishes to twist the quoted text at will and whim, based on their doctrinal-ideological agenda, making a totally inadmissible eisegesis: because it is clearly written in Rev.6:11 that those persons are given to each white robes and they are asked to 'rest for a while', ἀναπαύσονται, until the others would also die just like them. That is to say, when they rise their voices, they are already out of their bodies, and are given the white robes, the sign of their dignity, not of their annihilation, and asked to be at rest until fulfillment of time when all will suffer the same as they have suffered, i.e. death, that again means that they rise voice after that fact, i.e. the fact of death. Then why have we to quibble any longer whether they are alive/conscious or have disappeared with death? Nobody who disappears is given white robe, is soothed by a command to be reposed etc.
Below I shall give a history of the issue with some lucid arguments for repelling of heretical and soul-damaging and gloom-creating unbiblical ideas.
I.