Fortunately, EEOC considers national origin equally important to protect, and 'accentist' is likely adjacent enough that your lawyers will not support you going there.
Someone from deep rural alabama vs someone from illinois - they're both from the same country. both 'white', both have same qualifications. one has a deep accent that is hard to understand, one doesn't. Please define how making a decision based on accent would be 'racist' (by any EEOC definition).
Cursory research suggests that you would likely be liable for excluding an "ethnic group" if you refused to hire people with southern accent; "southerner" is an actual example I found in USG hiring guides.
again, if you have 2 candidates that have equal background, skills, etc, you have to make a choice if you can only hire one. it seems that any decision you make could be challenged on some point.
The 'alabama' accent might just be an accent they have, and they're not from alabama. But they still have an accent that is difficult for people to work with. If you have another candidate of equal experience and credentials, but who is easier to understand... do you hire the person who is harder to understand precisely because you might get sued otherwise?
