ffmpeg audio grid

ffmpeg -y -i 1.mp4 -i 2.mp4  -i 3.mp4 -i 4.mp4 -i 5.mp4  -i 6.mp4 -filter_complex "[0:a]volume=0.7[0a];[1:a]volume=0.7[1a];[2:a]volume=0.7[2a];[3:a]volume=0.7[3a];[4:a]volume=0.7[4a];[5:a]volume=0.7[5a];[0a][1a][2a][3a][4a][5a]amix=inputs=6[aout]" -map "[aout]" -c:a aac -b:a 128k out.aac

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Input #5, mov,mp4,m4a,3gp,3g2,mj2, from '6.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.25.101
  Duration: 00:09:21.71, start: 0.000000, bitrate: 4845 kb/s
    Stream #5:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yu
v420p, 960x540, 4520 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #5:1(und): Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, stereo, s16p, 31
9 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Output #0, adts, to 'out.aac':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.25.101
    Stream #0:0: Audio: aac (LC), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      encoder         : Lavc57.24.103 aac
Stream mapping:
  Stream #0:1 (mp3) -> volume
  Stream #1:1 (mp3) -> volume
  Stream #2:1 (mp3) -> volume
  Stream #3:1 (mp3) -> volume
  Stream #4:1 (mp3) -> volume
  Stream #5:1 (mp3) -> volume
  amix -> Stream #0:0 (aac)
Press [q] to stop, [?] for help
Error while filtering: Cannot allocate memory.3kbits/s speed=24.8x
size=    8989kB time=00:09:20.99 bitrate= 131.3kbits/s speed=24.8x
video:0kB audio:8823kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 1.871884%
[aac @ 00000000004fc8e0] Qavg: 668.345