Add background music to video while recording.

Hi, I want to accomplish a task: There is an audio link from server, app will play that audio using AVPlayer. User will press start to record video. After they finish recording the video, the audio will be inserted into the video already. There is just a task, and my budget for it is $100. Please add "abelhoang" to let me know you read the description. Don't bid if you don't understand or don't know how to do it. Note that user can use headphone while recording, so don't record the audio sound through microphone. ;). Remember I don't want to use AVMutableComposition to mix things after recording, because user will wait too long. I expect after user record video about 4 minutes, they just have to wait < 2 seconds to have the result. Thank you!