Backstory: Running is difficult and few people enjoy it, any motivation is helpful.
The Idea: An app that plays music clearly based on your average mile time
Deep Dive: A mobile app that would play music clearly based on how close you are to your average mile time. You could build a sort of progression that would pace users to run faster each week.
When you run significantly slower than the average time you set, the music would become distorted.
Validation: Nobody likes running. There are a ton of running apps out there that help pace and plan out your run times.
Monetization: You could charge a flat fee as there are little recurring costs for this app.