Converting .flac audio to .bnk format.
Converting Free Lossless Audio Codec Files directly to Bink Video Files (.flac to .bnk conversion) should be possible and typically relates to users attempting to import an audio track (FLAC) to a BNK video.
It appears that your options are limited and that your best bet could be using the Freemake Video Converter, which allows users to import FLAC audio to BNK videos.