With libsndfile there is no way to set a specific bitrate. Setting a specific constant bit rate it usually a bad idea anyway as the constant bitrate is likely to be too low for some parts of the sonf and too high for other parts.
With libsndfile you can however set various compression levels which always use variable bitrate. See : http://www.mega-nerd.com/libsndfile/command.html#SFC_SET_VBR_ENCODING_QUALITY