CreateAudioTrackForIndex
= fItem->GetTrackSupplier()->CreateAudioTrackForIndex(n);
AudioTrackSupplier* au = supplier->CreateAudioTrackForIndex(0);
virtual AudioTrackSupplier* CreateAudioTrackForIndex(int32 index);
virtual AudioTrackSupplier* CreateAudioTrackForIndex(int32 index) = 0;