The URL you provided with 127.0.0.1 as the redirect_uri worked fine for me. After I logged in and authorized a device, I was redirected to the localhost address provided with a URL parameter including the OAuth Code that needs to be exchanged for a token:
As of right now, I understand the advice is to create a SmartApp with End Points if you want to interact with devices. It sounds the like the SmartThings team has heard the feedback though, so I hope they are looking into alternative solutions.
—side note—
Depending on what kind of voice control you are looking for, I developed an app for Android called SharpTools which provides Widgets and Tasker Integration. And @jody.albritton is working on a web-based dashboard which has voice control features.
If you pair the SharpTools Tasker plugins with AutoVoice, you can easily setup voice commands like "Okay Google, turn the [device_name] on":