AVPlayer “freezes” the app at the start of buffering an audio stream

Don’t use playerItemWithURL it’s sync.
When you receive the response with the url try this:

AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:url options:nil];
NSArray *keys = @[@"playable"];

[asset loadValuesAsynchronouslyForKeys:keys completionHandler:^() {
    [self.player insertItem:[AVPlayerItem playerItemWithAsset:asset] afterItem:nil];
}];