Mix additional audio file with video(+audio) in ffmpeg - audio

Question

I'm trying to mix additional audio file with video which has also audio within. But the problem is that I already have complex ffmpeg command and don't know how to combine them together.
This is my existing ffmpeg command which uses some offsets and replaces additional audio file with embedded one (audio inside video) and also overlays few gauges and watermark to the video.
ffmpeg -y
-ss 00:00:01:213 -i videoFile.mp4
-ss 00:00:03:435 -i audioFile.wav
-i watermark.png
-framerate 6 -i gauge1_path/img-%04d.png
-framerate 1 -i gauge2_path/img-%04d.png
-framerate 2 -i gauge3_path/img-%04d.png
-framerate 2 -i gauge4_path/img-%04d.png
-framerate 2 -i gauge5_path/img-%04d.png
-framerate 2 -i gauge6_path/img-%04d.png
-filter_complex [0][2]overlay=(21):(H-h-21)[ovr0];
[ovr0][3]overlay=(W-w-21):(H-h-21)[ovr1];
[ovr1][4]overlay=(W-w-21):(H-h-333)[ovr2];
[ovr2][5]overlay=(W-w-21):(H-h-418)[ovr3];
[ovr3][6]overlay=(W-w-21):(H-h-503)[ovr4];
[ovr4][7]overlay=(W-w-21):(H-h-588)[ovr5];
[ovr5][8]overlay=(W-w-21):(H-h-673)
-map 0:v -map 1:a -c:v libx264 -preset ultrafast -crf 23 -t 00:5:10:000 output.mp4
Now I would like to use ffmpeg's amix in order to mix both audios instead of replacing them, if possible with ability to set volumes. But official documentation amix says nothing about volume.
Separately both seems to work ok.
ffmpeg -y -i video.mp4 -i audio.mp3 -filter_complex [0][1]amix=inputs=2[a] -map 0:v -map [a] -c:v copy output.mp4
and
ffmpeg -y -i video.mp4 -i audio.mp3 -i watermark.png -filter_complex [0][2]overlay=(21):(H-h-21)[ovr0] -map [ovr0]:v -map 1:a -c:v libx264 -preset ultrafast -crf 23 output.mp4
but together
ffmpeg -y -i video.mp4 -i audio.mp3 -i watermark.png -filter_complex [0][1]amix=inputs=2[a];[a][2]overlay=(21):(H-h-21)[ovr0] -map [ovr0]:v -map [a] -c:v libx264 -preset ultrafast -crf 23 output.mp4
I'm getting an error:
ffmpeg version N-93886-gfbdb3aa179 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 8.3.1 (GCC) 20190414
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
libavutil 56. 28.100 / 56. 28.100
libavcodec 58. 52.101 / 58. 52.101
libavformat 58. 27.103 / 58. 27.103
libavdevice 58. 7.100 / 58. 7.100
libavfilter 7. 53.101 / 7. 53.101
libswscale 5. 4.101 / 5. 4.101
libswresample 3. 4.100 / 3. 4.100
libpostproc 55. 4.100 / 55. 4.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
creation_time : 1970-01-01T00:00:00.000000Z
encoder : Lavf53.24.2
Duration: 00:00:29.57, start: 0.000000, bitrate: 1421 kb/s
Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1032 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
creation_time : 1970-01-01T00:00:00.000000Z
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, 5.1, fltp, 383 kb/s (default)
Metadata:
creation_time : 1970-01-01T00:00:00.000000Z
handler_name : SoundHandler
[mp3 # 0000015e2f934ec0] Estimating duration from bitrate, this may be inaccurate
Input #1, mp3, from 'audio.mp3':
Duration: 00:00:45.33, start: 0.000000, bitrate: 128 kb/s
Stream #1:0: Audio: mp3, 44100 Hz, stereo, fltp, 128 kb/s
Input #2, png_pipe, from 'watermark.png':
Duration: N/A, bitrate: N/A
Stream #2:0: Video: png, rgb24(pc), 100x56 [SAR 3779:3779 DAR 25:14], 25 tbr, 25 tbn, 25 tbc
[Parsed_amix_0 # 0000015e2ff2e940] Media type mismatch between the 'Parsed_amix_0' filter output pad 0 (audio) and the 'Parsed_overlay_1' filter input pad 0 (video)
[AVFilterGraph # 0000015e2f91c600] Cannot create the link amix:0 -> overlay:0
Error initializing complex filters.
Invalid argument
So my question: whether it's possible to combine amix and overlay together and how and in which order they should be used? Or should I look something different because amix unable to set volume levels?
Thanks in advance!

Answer 1

Use
ffmpeg -y -i video.mp4 -i audio.mp3 -i watermark.png -filter_complex [0][1]amix=inputs=2,volume=2[a];[0][2]overlay=(21):(H-h-21)[ovr0] -map [ovr0]:v -map [a] -c:v libx264 -preset ultrafast -crf 23 output.mp4
You're sending the mixed audio to the overlay filter which requires video input. Overlay should be fed the original video stream. The audio output [a] should left alone. It is consumed as a mapped output stream.
volume filter added after amix to restore volume. amix reduces volume, in order to avoid clipping.