If Chapter 1 of Midchildan Music was the introduction of the Sergeant, Chapter 2 should by all rights be the introduction of Miku, and it is named Vocaloid Device after all. Logically, then, it would be about the Vocaloid in the story.
This is all a very long way of getting around to saying "that isn't how chapter 2 turned out in the end."
The opening and ending to the chapter are about Miku, but they're about Miku as an Intelligent Device, not as a character. I wish I could claim some grand significance to that gesture, but honestly when I was writing it it was more a matter of "how do I characterize Miku?" With the difficulty I had in answering that question, I ended up minimizing Miku's "character time" rather than solidly defining this Miku, and that's an error I plan to rectify with the next chapter.
In the end, the answer that I've come to is that the Vocaloids are expressed through the music that they sing. There's no anime to provide a look at what the Vocaloids are like, character-wise. The fandom has assigned certain characteristics to many of the Vocaloids... but in the end, I feel like it comes down to "what songs are you going to use as the guide to their characters?" (If anyone has better thoughts on how to characterize the Vocaloids, do let me know in the comments.)
And considering how many songs there are for Miku, that pretty much leaves her wide-open, character-wise. I'm mostly familiar with the songs from the Project Diva video games, and so that's what I'm generally using to define who "Miku" is. But that's more a matter for next chapter, when Miku gets to talk more about what she wants.
As long as I'm talking about songs, let me mention that I do know what she sang on the training field. I realize that I provided very little to identify it by; that was intentional. As was the "different language" bit, which was partially a slight callback to Nanoha (all the characters speak Japanese, Midchildan devices speak English, Belkan devices speak German, and somehow all the characters can fully understand their devices) and also a way to hide from the Sergeant what Miku was singing. It's not yet time for him to fully understand just what he's gotten himself into as far as Miku's personality is concerned.
Really, that's because even at the end of this chapter, he's still not quite as "aware" of how he should treat Miku as he needs to be. With his CO's help (sadly, Major Nakamura and her Final Judgment are probably not going to play much more of a role in this story) he's realized that an Intelligent Device is different from a storage device, but as he himself mentioned, there's one more category of device to worry about, one that's orders of magnitude different from even an Intelligent Device.
I am a little disappointed with myself for devoting so little time to this chapter's shift in the Sergeant's personality. He does come around very quickly, and I'm left wondering if perhaps he shouldn't have been quite so abrasive at first, or perhaps if it should have taken longer to get him to change his attitude...
Well, I don't regret making him somewhat abrasive and short with people in general, and I'd like to hope that he is, if not particularly sympathetic, at least understandable in his personality. And although it may look a little unlikely now, in the end I will try to forge a bond between Miku and the Sergeant. It won't be a romantic one, but then bonds between people don't have to be romantic. And with any luck, the next few chapters will get the Sergeant to a point where that bond won't cause people to throw their keyboards through the screen yelling "why is she still giving him the time of day".
... Hm. There's a lot of foreshadowing future events in this, and less talking about the chapter. I guess I did say I liked foreshadowing...