A sound channel does not reset its position automatically. You have to stop it playing and reset its position manually. So do what zyblade wrote with the change that you set the position to zero prior to playing the channel.
+ On collision between Ball and Brick
-> XAudio2: Set Channel 1 Position to 0 seconds
-> XAudio2: Play Channel 1
Edit: I'm sorry, you also have to add this line to your event sheet:
+ XAudio2: [negated] Channel 1 is playing
-> XAudio2: Stop channel 1