Make a music bed or one short sound hit for an edit (the beat-and-hits editor workflow), with no voice and no words. kind "bed" gives an instrumental bed of about seconds (4 to 60, default 15) to run under the piece; kind "hit" gives one sub-second sound to place on every cut. Pass description in plain words: the feel, tempo, instruments, texture. It is charged through the same audio generation as generate_audio (a token quote applies per second) and the file is saved to your library. Returns audio_url, duration_seconds and generation_id.