Displaying Artwork for .MP3 file

Change your snippet of code into this (I already tested it):

I added println lines commented in places of interest, Feel free to uncomment in order to see what is happening.

   for item in metadataList {
        if item.commonKey == nil{
            continue
        }

        if let key = item.commonKey, let value = item.value {
            //println(key)
            //println(value)
            if key == "title" {
                trackLabel.text = value as? String
            }
            if key  == "artist" {
                artistLabel.text = value as? String
            }
            if key == "artwork" {
                if let audioImage = UIImage(data: value as! NSData) {
                  //println(audioImage.description)
                    artistImage.image = audioImage
                }
            }
        }
    }

UPDATE: A bit of clean up of this code

for item in metadataList {

    guard let key = item.commonKey, let value = item.value else{
        continue
    }

   switch key {
    case "title" : trackLabel.text = value as? String
    case "artist": artistLabel.text = value as? String
    case "artwork" where value is NSData : artistImage.image = UIImage(data: value as! NSData)
    default:
      continue
   }
}

UPDATE: For Swift 4

for item in metadataList {

    guard let key = item.commonKey?.rawValue, let value = item.value else{
        continue
    }

   switch key {
    case "title" : trackLabel.text = value as? String
    case "artist": artistLabel.text = value as? String
    case "artwork" where value is Data : artistImage.image = UIImage(data: value as! Data)
    default:
      continue
   }
}