ffmpeg -i RTb295d0534191e1acb22a45bb971a12e6.mka -i RT103bfe5f4b129860f69cd8e820f3a10b.mka -filter_complex "[1:a]adelay=13500s:all=1[apad]; [0:a][apad]amix=inputs=2:weights=1|1[aout]" -map [aout] combined_audio.mka
ffmpeg version n5.0-4-g911d7f167c-20220311 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 11.2.0 (crosstool-NG 1.24.0.533_681aaef)
  configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static --pkg-config=pkg-config --cross-prefix=x86_64-w64-mingw32- --arch=x86_64 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug --disable-w32threads --enable-pthreads --enable-iconv --enable-libxml2 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp --enable-lzma --enable-fontconfig --enable-libvorbis --enable-opencl --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib --enable-amf --enable-libaom --enable-avisynth --enable-libdav1d --enable-libdavs2 --disable-libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-libgme --enable-libass --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librist --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2 --enable-libmfx --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-libsoxr --enable-libsrt --enable-libsvtav1 --enable-libtwolame --enable-libuavs3d --disable-libdrm --disable-vaapi --enable-libvidstab --enable-vulkan --enable-libshaderc --enable-libplacebo --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libzimg --enable-libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-version=20220311
  libavutil      57. 17.100 / 57. 17.100
  libavcodec     59. 18.100 / 59. 18.100
  libavformat    59. 16.100 / 59. 16.100
  libavdevice    59.  4.100 / 59.  4.100
  libavfilter     8. 24.100 /  8. 24.100
  libswscale      6.  4.100 /  6.  4.100
  libswresample   4.  3.100 /  4.  3.100
  libpostproc    56.  3.100 / 56.  3.100
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
Stream mapping:
  Stream #0:0 (opus) -> amix
  Stream #1:0 (opus) -> adelay:default
  amix:default -> Stream #0:0 (libvorbis)
Press [q] to stop, [?] for help
Output #0, matroska, to 'combined_audio.mka':
  Metadata:
    encoder         : Lavf59.16.100
  Stream #0:0: Audio: vorbis (oV[0][0] / 0x566F), 48000 Hz, stereo, fltp
    Metadata:
      encoder         : Lavc59.18.100 libvorbis
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time231x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time184x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time189x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time223x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time275x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time245x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time213x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time209x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time208x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time204x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time199x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time193x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time185x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time181x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time178x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time177x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time176x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time169x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time167x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time163x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time146x
[libvorbis @ 00000229f8a7bbc0] Queue input is backward in time139x
size=   75141kB time=06:07:52.57 bitrate=  27.9kbits/s speed= 130x
video:0kB audio:70470kB subtitle:0kB other streams:0kB global headers:4kB muxing overhead: 6.628071%