There have been a couple of cases of people reporting that the MBC app makes no external sound on iPad- but works fine with headphones. If you find yourself in this situation the problem may be solved by one of the following:
1. double tap the home button, slide to the right and on the left side there will be the "unmute" button. Toggle that and see if it makes a difference.
2. More complex instructions involve: going to settings, changing the iPad top right switch from rotation to mute, then actually use that top switch to unmute the iPad, and then try the sound. I imagine after that you can set it back to rotation (assuming it's currently controlling rotation).
Step 2 might work if the problem is on an iPhone/iPad - but I've only had confirmation that this solves the problem on iPad. The only clue I've had that sound may be a problem on iPhone is a 1 star review on the iTunes store with no opportunity to follow that up...