In dulce et decorum est, is it the poet's voice speaking or is it the voice of somebody else, real or imagined?Just that who is speaking in the poem and whether they are real or imagined.
This is a good question, since in poetry it is often difficult to tell whose voice is speaking. We usually call the voice speaking in the poen the "persona" and many students make the mistake of thinking that the persona is always the poet. I am glad to see that you are thinking more deeply about the persona.
Since the poem is written in the first person plural (i.e., "We" is used), the reader assumes that the persona is someone wh has been through the experiences in the poem, namely World War One's gas/chemical warfare. Since the poet had personally experienced World War One and in fact died before the war was over, the persona could very well be speaking in his voice. But a more powerful interpretation could also be thatthe persona represents any soldier from that war, or even any soldier from any war.
In the last verse, the persona addresses "you", which is usually understood to be the reader or the world at large through extension from the reader. This would indicate that the persona has a message for the world--that all the bravado in the phrase dulce et decorum est pro partia mori cannot really compensate for the horrors of actual combat.
One interpretation of this poem that would indicate that the poet, Owen, was using his own voice as the persona's is that he dedicated the poem to Jessie Pope, who was a fellow World War One era poet. Unlike Owen, she never actually participated in combat herself. Also unlike Owen, her poetry was very pro-war, encouraging young men to join the fighting.
In analyzing poetry, there are many factors to consider when thnking of persona/voice. Each interpretation, while never 100% "correct", offers different ways to understand the poet's intended message.
Of course it is impossible to know for sure whether Wilfred Owen meant for us to think that he himself is the voice in the poem. However, it does seem likely that it is his voice rather than some other person, real or imagined.
Throughout the poem, the narrator refers to himself as "me" and refers to the unit of soldiers as "we." But the poet could have just been doing that for effect.
More importantly, we know that Owen himself went through many of the things that he describes in the poem. Before he was killed, Owen participated in the trench warfare of WWI and would have seen many horrors, including gas attacks. So we can say that it is likely that the speaker in the poem is Owen himself.