So even though Bede tells us that Caedmon sang this hymn to an angel in a barn, we can't assume that Caedmon and the actual speaker of the poem are the same. Why's that you say? Well, for one thing, the speaker talks in the first-person plural (technical term for "we"), which automatically means that s/he is more than just one person.
What's even more interesting is that you, the reader, are included in that "we." Think about the first line: "Now we must praise heaven-kingdom's Guardian" (1). The speaker may be the guy reciting the poem, but anyone listening is ordered to join in the praising. We are drawn into the hymn, becoming part of the community that is listing and celebrating all of God's glorious deeds.
In fact, because the speaker speaks in such enormous, universal terms—"men's sons" and "mankind"—he seems to be voicing the opinions of the entire population. Granted, that wasn't a whole lot in the year 680, at least compared to our modern population of nearly 7 billion. But still, it takes moxie to relate the admiration of the whole world.
This emphasizes the universality of the hymn. After all, if God "first created for men's sons Heaven as a roof" and made them "middle-earth," it makes sense for everyone enjoying the grass and sky to say thank you. This means that even if we do identify Caedmon as the speaker, he is talking on behalf of everyone, us included.
Notice, however, that after a strong showing in the first line, the "we" drops out for the next eight lines. Why do you think this happens?