ffmpeg -i RTb295d0534191e1acb22a45bb971a12e6.mka -i RT103bfe5f4b129860f69cd8e820f3a10b.mka -filter_complex "[0:a]asetpts=PTS-STARTPTS[a0set]; [1:a]asetpts=PTS-STARTPTS[a1set];[a1set]adelay=13500s:all=1[apad]; [a0set][apad]amix=inputs=2:weights=1|1[aout]" -map [aout] combined_audio.mka
ffmpeg version 2022-03-31-git-e301a24fa1-essentials_build-www.gyan.dev Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 11.2.0 (Rev7, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-zlib --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-sdl2 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libgme --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libtheora --enable-libvo-amrwbenc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-librubberband
  libavutil      57. 24.101 / 57. 24.101
  libavcodec     59. 25.100 / 59. 25.100
  libavformat    59. 20.101 / 59. 20.101
  libavdevice    59.  6.100 / 59.  6.100
  libavfilter     8. 29.100 /  8. 29.100
  libswscale      6.  6.100 /  6.  6.100
  libswresample   4.  6.100 /  4.  6.100
  libpostproc    56.  5.100 / 56.  5.100
Input #0, matroska,webm, from 'RTb295d0534191e1acb22a45bb971a12e6.mka':
  Metadata:
    encoder         : GStreamer matroskamux version 1.16.2
    creation_time   : 2022-03-23T21:20:27.000000Z
  Duration: 03:45:00.47, start: 0.291000, bitrate: 19 kb/s
  Stream #0:0(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      title           : Audio
Input #1, matroska,webm, from 'RT103bfe5f4b129860f69cd8e820f3a10b.mka':
  Metadata:
    encoder         : GStreamer matroskamux version 1.16.2
    creation_time   : 2022-03-24T01:05:30.000000Z
  Duration: 02:45:03.51, start: 13502.587000, bitrate: 5 kb/s
  Stream #1:0(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      title           : Audio
File 'combined_audio.mka' already exists. Overwrite? [y/N] y
Stream mapping:
  Stream #0:0 (opus) -> asetpts:default
  Stream #1:0 (opus) -> asetpts:default
  amix:default -> Stream #0:0 (libvorbis)
Press [q] to stop, [?] for help
Output #0, matroska, to 'combined_audio.mka':
  Metadata:
    encoder         : Lavf59.20.101
  Stream #0:0: Audio: vorbis (oV[0][0] / 0x566F), 48000 Hz, stereo, fltp
    Metadata:
      encoder         : Lavc59.25.100 libvorbis
[libvorbis @ 0000013473fce140] Queue input is backward in time
[libvorbis @ 0000013473fce140] Queue input is backward in time235x
[libvorbis @ 0000013473fce140] Queue input is backward in time195x
[libvorbis @ 0000013473fce140] Queue input is backward in time196x
[libvorbis @ 0000013473fce140] Queue input is backward in time223x
[libvorbis @ 0000013473fce140] Queue input is backward in time277x
[libvorbis @ 0000013473fce140] Queue input is backward in time249x
[libvorbis @ 0000013473fce140] Queue input is backward in time216x
[libvorbis @ 0000013473fce140] Queue input is backward in time214x
[libvorbis @ 0000013473fce140] Queue input is backward in time210x
[libvorbis @ 0000013473fce140] Queue input is backward in time207x
[libvorbis @ 0000013473fce140] Queue input is backward in time202x
[libvorbis @ 0000013473fce140] Queue input is backward in time195x
[libvorbis @ 0000013473fce140] Queue input is backward in time189x
[libvorbis @ 0000013473fce140] Queue input is backward in time183x
[libvorbis @ 0000013473fce140] Queue input is backward in time180x
[libvorbis @ 0000013473fce140] Queue input is backward in time179x
[libvorbis @ 0000013473fce140] Queue input is backward in time178x
[libvorbis @ 0000013473fce140] Queue input is backward in time171x
[libvorbis @ 0000013473fce140] Queue input is backward in time170x
[libvorbis @ 0000013473fce140] Queue input is backward in time166x
[libvorbis @ 0000013473fce140] Queue input is backward in time148x
[libvorbis @ 0000013473fce140] Queue input is backward in time141x
size=   75141kB time=06:07:52.57 bitrate=  27.9kbits/s speed= 131x
video:0kB audio:70470kB subtitle:0kB other streams:0kB global headers:4kB muxing overhead: 6.628052%