Andy, here is the format block for MP3

off:00 WORD    wFormatTag;               
off:02 WORD    nChannels;         
off:04 DWORD   nSamplesPerSec;    
off:08 DWORD   nAvgBytesPerSec;   
off:0C WORD    nBlockAlign;       
off:0E WORD    wBitsPerSample;    
off:10 WORD    cbSize;                                               
off:12 WORD    wID;
off:14 DWORD   fdwFlags;
off:18 WORD    nBlockSize;
off:1A WORD    nFramesPerBlock;
off:1C WORD    nCodecDelay;

#define MPEGLAYER3_ID_UNKNOWN            0
#define MPEGLAYER3_ID_MPEG               1
#define MPEGLAYER3_ID_CONSTANTFRAMESIZE  2

#define MPEGLAYER3_FLAG_PADDING_ISO      0x00000000
#define MPEGLAYER3_FLAG_PADDING_ON       0x00000001
#define MPEGLAYER3_FLAG_PADDING_OFF      0x00000002