I have hundreds of videos of karaoke in 5.1 aac. I would like to delete the first channel which is voice. I'm using the following command but it seems that the video is being reencoded:

ffmpeg -i in.mp4 -af "pan=5.1| c1=c1 | c2=c2 | c3=c3 | c4=c4 |c5=c5" -vcodec copy out.mp4

With the commands bellow it's instantly:

ffmpeg -i in.mp4 -af "pan=5.1| c1=c1 | c2=c2 | c3=c3 | c4=c4 |c5=c5" out.mp3
ffmpeg -i in.mp4 -acodec copy -vcodec copy out.mp4

Here's the result:

ffmpeg version 2023-12-14-git-5256b2fbe6-full_build-www.gyan.dev Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.0 (Rev10, Built by MSYS2 project)

  libavutil      58. 33.100 / 58. 33.100
  libavcodec     60. 35.100 / 60. 35.100
  libavformat    60. 18.100 / 60. 18.100
  libavdevice    60.  4.100 / 60.  4.100
  libavfilter     9. 14.100 /  9. 14.100
  libswscale      7.  6.100 /  7.  6.100
  libswresample   4. 13.100 /  4. 13.100
  libpostproc    57.  4.100 / 57.  4.100
[aist#0:1/aac @ 000001f224c97c00] Guessed Channel Layout: 5.1
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'in.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 512
    compatible_brands: mp42iso2avc1mp41
    creation_time   : 2023-12-17T21:54:01.000000Z
    encoder         : HandBrake 1.7.1 2023112200
  Duration: 00:04:07.43, start: 0.000000, bitrate: 1720 kb/s
  Chapters:
    Chapter #0:0: start 0.000000, end 0.500000
      Metadata:
        title           : Chapter 1
    Chapter #0:1: start 0.500000, end 247.333000
      Metadata:
        title           : Chapter 2
  Stream #0:0[0x1](und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709, progressive), 708x480 [SAR 8:9 DAR 59:45], 1583 kb/s, 30 fps, 30 tbr, 90k tbn (default)
    Metadata:
      creation_time   : 2023-12-17T21:54:01.000000Z
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, 6 channels, fltp, 128 kb/s (default)
    Metadata:
      creation_time   : 2023-12-17T21:54:01.000000Z
      handler_name    : Surround
      vendor_id       : [0][0][0][0]
  Stream #0:2[0x3](eng): Data: bin_data (text / 0x74786574), 0 kb/s
    Metadata:
      creation_time   : 2023-12-17T21:54:01.000000Z
      handler_name    : SubtitleHandler

Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[Parsed_pan_0 @ 000001f224c46c00] Pure channel mapping detected: M 1 2 3 4 5
Output #0, mp4, to 'out.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 512
    compatible_brands: mp42iso2avc1mp41
    encoder         : Lavf60.18.100
  Chapters:
    Chapter #0:0: start 0.000000, end 0.500000
      Metadata:
        title           : Chapter 1
    Chapter #0:1: start 0.500000, end 247.333000
      Metadata:
        title           : Chapter 2
  Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709, progressive), 708x480 [SAR 8:9 DAR 59:45], q=2-31, 1583 kb/s, 30 fps, 30 tbr, 90k tbn (default)
    Metadata:
      creation_time   : 2023-12-17T21:54:01.000000Z
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]
  Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, 5.1, fltp, 341 kb/s (default)
    Metadata:
      creation_time   : 2023-12-17T21:54:01.000000Z
      handler_name    : Surround
      vendor_id       : [0][0][0][0]
      encoder         : Lavc60.35.100 aac
[out#0/mp4 @ 000001f224c8e2c0] video:47820kB audio:4411kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.504104%
size=   52495kB time=N/A bitrate=N/A speed=N/A
[aac @ 000001f224f40840] Qavg: 65536.000