现在我可以从音频文件中获取这些标签：
#define kAFInfoDictionary_Artist "artist"
#define kAFInfoDictionary_Album "album"
#define kAFInfoDictionary_Tempo "tempo"
#define kAFInfoDictionary_KeySignature "key signature"
#define kAFInfoDictionary_TimeSignature "time signature"
#define kAFInfoDictionary_TrackNumber "track number"
#define kAFInfoDictionary_Year "year"
#define kAFInfoDictionary_Composer "composer"
#define kAFInfoDictionary_Lyricist "lyricist"
#define kAFInfoDictionary_Genre "genre"
#define kAFInfoDictionary_Title "title"
#define kAFInfoDictionary_RecordedDate "recorded date"
#define kAFInfoDictionary_Comments "comments"
#define kAFInfoDictionary_Copyright "copyright"
#define kAFInfoDictionary_SourceEncoder "source encoder"
#define kAFInfoDictionary_EncodingApplication "encoding application"
#define kAFInfoDictionary_NominalBitRate "nominal bit rate"
#define kAFInfoDictionary_ChannelLayout "channel layout"
#define kAFInfoDictionary_ApproximateDurationInSeconds "approximate duration in seconds"
#define kAFInfoDictionary_SourceBitDepth "source bit depth"
#define kAFInfoDictionary_ISRC "ISRC" // International Standard Recording Code
#define kAFInfoDictionary_SubTitle "subtitle"
那么如何获得Disc Number标签？谢谢！