http://uk.un4seen.com/files/bass23.zip

HSTREAM BASS_StreamCreate(
    DWORD freq,
    DWORD chans,
    DWORD flags,  // BASS_SAMPLE_FLOAT |BASS_SAMPLE_SOFTWARE 
    STREAMPROC *proc,
    DWORD user
); 

DWORD CALLBACK YourStreamProc(
    HSTREAM handle,
    void *buffer,
    DWORD length,
    DWORD user
);

Parameters
handle The stream that needs writing. 
buffer The buffer to write the sample data in 32-bit floating-point samples range from -1 to +1. 
length The number of BYTES to write. 
user The user instance data given when BASS_StreamCreate was called.