I only have my experiences and those of people choose to me, but I don't see this working because:

1. I think people who use GPS rely on the visual side moreso than the audio side.
2. I think people who walk home safely at night in urban areas tend to be more street smart. Those people already have a familiar route. Taking a new route in the dark seems like a recipe for disaster. People flub GPS directions all the time.
3. I think the people who are uncomfortable walking are going to call an Uber.
4. I think people who are uncomfortable both walking and using Uber at night aren't the type of people to be going out at night anyway.
5. I think people already uncomfortable going out in the dark at night aren't going to be suddenly more comfortable.
6. If it were to take off on a large scale, criminals would just use the app to see what routes people walk. I would just put in a popular bar address and a college campus address and I know what route is "safest".

I think your heart is in the right place but I just don't see it working out. Good luck to you, but I'd take your app development skills and try a different app.

All is my opinion, though

Walking at night face down in your phone instead of paying attention to your surroundings seems like the opposite of staying safe to me.