What do you mean the voice on TextEdit? If you select the Speech->Start Speaking, that's a system wide thing, and can be changed in
System Preferences/Settings -> Accessibility -> Speak Selection -> Voice. You can hit Manage to get a big list of voices with Enhanced and Premium versions to pick from, and set a keyboard shortcut that makes any selected text from anywhere be read with the selected voice. I personally use the Alex voice at maximum speed and use it for reading any longer texts (visually impaired myself). Some of the others sound more natural and have more speed options but I've grown so used to the Alex voice over my many years of Mac use