Mr. Sweeney is Sophie's High School teacher (there's not too much information about if she has other teachers or not) who has taken his class on a field trip to the Natural History Museum in Balboa park. I'd say his accent is probably an American accent, considering he lives in California, but really any accent would work since I don't know his background.

His voice is supposed to be nasally, if you can't do that it's fine, but if you can that would be better.