I'm trying to get a idea of what various northern U.S. accents may have sounded like in the mid-19th Century, and I was wondering if there are any "oral history" recordings of northern/Union Civil War veterans? All of the recordings I've found thus far are either of southern/Confederate veterans or former slaves.
It's very difficult to research the history of U.S. English without getting completely inundated with either people asking if Americans really spoke like Cary Grant prior to ~1965, or people claiming that an Elizabethan-era Englishman would have sounded indistinguishable from Foghorn Leghorn.
You may be interested in this previous post of mine which contains a collection of sound recordings of Union veterans as well as some more recordings of Northerners who were alive at the time of the Civil War.
Maybe someone can add a bit more on your question of "oral histories" of Northerners. The only ones I know of are the ones I linked to. Albert Woolson, the last living veteran, is the only Union soldier I know of that was interviewed at any length, and by that time, he was already more than 100 years old, so his voice is rather frail. The others are all a few minutes long, if that.
There may be some more than these, just not online. If anybody knows of any others, I'd be interested to hear about them too!
If the topic interests you, you may also want to read the top post of that thread linked above. The one with the Civil War recordings was a follow-up to a question about Southerns vs. Northern accents in the 18th and into the 19th centuries.