While there's no audio recording of them speaking, their letters and interviews from the time period suggest a regional dialect typical of the Midwest. They likely had:
* A flat, neutral tone - common in the Midwest.
* Slightly drawn-out vowels - characteristic of the region.
* Use of "pop" instead of "soda" - a common Midwestern colloquialism.
However, it's important to remember that accents are fluid and vary even within a region. They would have likely sounded different from each other, too.