I have discussed running cadence before, and the importance of working with cadence to try to reduce musculoskeletal stress, muscle effort, and strain on the muscles, ligaments, and joints.
(See link 
Running - cadence)
Here is an app that let you pick a cadence, and then tune your music to the cadence.
Audiostep
Audiostep is an iPhone app that lets you pick a preferred cadence from anywhere between 120-200 steps per minute. It has a slider that lets you adjust.
Then the app makes all of your music adjust into that tempo range.
It has normalized music volume, it supports Runkeeper, and has support for HR warnings if you want. And it has voice status updates for distance, time, avg pace, etc.
The problem is that it only accesses music in your phone’s music library. It does not sync with music apps like XM. 
That kinda sucks.
$0.99 from Audioforge Labs Inc.
If you don’t want to mess with your music and change the tempo of it, or if you just want to hear cadence beat reminders, there are other options. You can just use a cadence feature on your watch, of course. Or you can get a phone app.
There are 10 or 100 apps that just play a metronome in the background, to help you try to hit whatever cadence you want. Maybe you have already used some, or have a favorite.
Since there are so many of them, and I have not tried them all, I will just share the one I personally use and like.
I use Tempo.
Tempo
Tempo is NOT a running app, it is a music app. But it is extremely adjustable, and makes a very audible and distinct sound, so it works great as a running app. It is the number one metronome app used by musicians. It has a number of functions that you won’t need for running, but the thing I like about it is that it will play over whatever music you are playing. And the sound is distinct, so you can clearly focus on it for cadence. You can adjust to make the beat match for a single leg stride or for both legs, whichever you prefer.
(It has a tempo range of 10 to 800. Try running that!)