Do you mean converting lyrics from sound to text, or just getting lyrics that are already in some kind of tag? If the latter, there are Lyrics3v2 tags specifically for lyrics, but I think ID3v2 tags are more commonly used. Both types of tag are available from BASS_ChannelGetData, with BASS_TAG_LYRICS3 and BASS_TAG_ID3V2, respectively. The returned tag block will need to be processed further to extract the wanted info, eg. using something like id3lib. Details on both tags can usually be found on the ID3 website, but that seems to be down currently (hopefully back soon).
https://id3.org
