Please follow below steps to hear trainer over music.
  1. Open app
  2. Click Settings
  3. Press Workout Options
  4. Set Trainer Volume to maximum
Note: iOS couch to 5k app users do not need to follow above steps but adjust phone volume to hear.

Note: Please set your phone volume to proper level. If you still could not hear audio cues after app goes to background, try to tap the lock icon in the upper left corner of the workout screen or change to another trainer.