Andy, here is the format block for MP3

WORD    wFormatTag;        
WORD    nChannels;         
DWORD   nSamplesPerSec;    
DWORD   nAvgBytesPerSec;   
WORD    nBlockAlign;       
WORD    wBitsPerSample;    
WORD    cbSize;                                               
WORD    wID;
DWORD   fdwFlags;
WORD    nBlockSize;
WORD    nFramesPerBlock;
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