Setting playerItem.audioMix while AVPlayer is actively playing causes
the audio/video pipeline to reconfigure, stalling video playback.
Fix: install the MTAudioProcessingTap audioMix at item creation time
(in playIndex, before replaceCurrentItem), so the pipeline is already
configured when playback begins. startRecording now only flips the
isRecording flag and skips re-installation if audioMix exists.
Also removed the guard that blocked tap installation when tracks
weren't available yet (HLS streams) — the wildcard trackID
(kCMPersistentTrackID_Invalid) handles this case.
yumoqing
9160b3ce2b