$ ffmpeg -hide_banner -i input.flac -f wav - | wavpack -i - audio.wv

when stdin is used for input, it must be the only file!                                

can't ignore length in header when both input and output are pipes, '-y' to override                                

type 'wavpack' for short help or 'wavpack --help' for full help
Input #0, flac, from 'input.flac':
  Metadata:
    TITLE           : SONG OF SAYA I
    ARTIST          : Nitro+
    ALBUM           : Saya no Uta Original Soundtrack
    track           : 5
    GENRE           : Soundtrack
    DATE            : 2004
  Duration: 00:03:26.81, start: 0.000000, bitrate: 885 kb/s
  Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, wav, to 'pipe:':
  Metadata:
    INAM            : SONG OF SAYA I
    IART            : Nitro+
    IPRD            : Saya no Uta Original Soundtrack
    IPRT            : 5
    IGNR            : Soundtrack
    ICRD            : 2004
    ISFT            : Lavf61.7.100
  Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, stereo, s16, 1411 kb/s
      Metadata:
        encoder         : Lavc61.19.100 pcm_s16le
[aost#0:0/pcm_s16le @ 0x5558f4b1d250] Error submitting a packet to the muxer: Broken pipe
    Last message repeated 1 times
[out#0/wav @ 0x5558f4b1cb10] Error muxing a packet
[out#0/wav @ 0x5558f4b1cb10] Task finished with error code: -32 (Broken pipe)
[out#0/wav @ 0x5558f4b1cb10] Terminating thread with return code -32 (Broken pipe)
[out#0/wav @ 0x5558f4b1cb10] Error writing trailer: Broken pipe
[out#0/wav @ 0x5558f4b1cb10] Error closing file: Broken pipe
[out#0/wav @ 0x5558f4b1cb10] video:0KiB audio:4KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 4.383681%
size=       5KiB time=00:00:00.18 bitrate= 210.4kbits/s speed= 205x    
Conversion failed!