If you have iOS 5 and no longer hear your audio cues when your screen is locked, there’s an easy fix for this.
iOS 5 is treating things a little differently in terms of notifications, which include audio cues in our Run apps. To get your audio cues working again, you need to make sure Run 5k (or 10k) is listed in the Notifications Center on your phone. Go into your Settings app on your device and go to Notifications. Scroll down and tap on the app. Then, turn on Notification Center, choose an alert style, and turn everything else on (Sound, Badge App Icon, Lock Screen). Your audio cues should work just as they did before. Now back to running!