converting eac3 to aac with ffmpeg - ffmpeg

Question

I have a number of video files stored on my Synology of which the audio doesn't play on when opening them with DS File. This is because DS File does not support eac3. I would like to convert those files to aac using ffmpeg, but when doing so, all audio is lost.
This is the file info:
Input #0, matroska,webm, from '<file>.mkv':
Metadata:
encoder : libebml v1.4.2 + libmatroska v1.6.4
Duration: 00:48:43.42, start: 0.000000, bitrate: 6566 kb/s
Stream #0:0: Video: h264 (High), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
Stream #0:1(eng): Audio: eac3, 48000 Hz, 5.1 (default)
Stream #0:2(eng): Subtitle: subrip
Metadata:
title : English [SDH]
Stream #0:3(ara): Subtitle: subrip
Metadata:
title : Arabic
Stream #0:4(chi): Subtitle: subrip
This is my attempt:
ffmpeg -i <file>.mkv -map 0:v -map 0:a:0 -map 0:s -c copy -c:a aac -b:a 640k output.mkv
Subtitles are kept, but audio is completely removed. I would like to automate the process if possible.
This is my ffmpeg configuration:
ffmpeg version 4.1.8 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 8.5.0 (GCC)
configuration: --prefix=/usr --incdir='${prefix}/include/ffmpeg' --arch=i686 --target-os=linux --cross-prefix=/usr/local/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu- --enable-cross-compile --enable-optimizations --enable-pic --enable-gpl --enable-shared --disable-static --disable-stripping --enable-version3 --enable-encoders --enable-pthreads --disable-protocols --disable-protocol=rtp --enable-protocol=file --enable-protocol=pipe --disable-muxer=image2 --disable-muxer=image2pipe --disable-swscale-alpha --disable-ffplay --disable-ffprobe --disable-doc --disable-devices --disable-bzlib --disable-altivec --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libmp3lame --disable-vaapi --disable-cuvid --disable-nvenc --disable-decoder=aac --disable-decoder=aac_fixed --disable-encoder=aac --disable-decoder=amrnb --disable-decoder=ac3 --disable-decoder=ac3_fixed --disable-encoder=zmbv --disable-encoder=dca --disable-decoder=dca --disable-encoder=ac3 --disable-encoder=ac3_fixed --disable-encoder=eac3 --disable-decoder=eac3 --disable-encoder=truehd --disable-decoder=truehd --disable-encoder=hevc_vaapi --disable-decoder=hevc --disable-muxer=hevc --disable-demuxer=hevc --disable-parser=hevc --disable-bsf=hevc_mp4toannexb --x86asmexe=yasm --cc=/usr/local/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-wrap-gcc --enable-yasm --enable-libx264 --enable-encoder=libx264
libavutil 56. 22.100 / 56. 22.100
libavcodec 58. 35.100 / 58. 35.100
libavformat 58. 20.100 / 58. 20.100
libavdevice 58. 5.100 / 58. 5.100
libavfilter 7. 40.101 / 7. 40.101
libswscale 5. 3.100 / 5. 3.100
libswresample 3. 3.100 / 3. 3.100
libpostproc 55. 3.100 / 55. 3.100
Synology does not support eac3 out of the box. So is this even possible?

Answer 1

Try adding channels for audio as well along side bit rate to the command line after aac.
for 5.1 audio add:
-ac 6 -b:a 384k
For stereo audio add:
-ac 2 -b:a 128k
The final command would be like
ffmpeg -i <file>.mkv -map 0:v -map 0:a:0 -map 0:s -c copy -c:a aac -ac 6 -b:a 640k output.mkv

Answer 2

Found it, I was apparently using the built-in ffmpeg, while I should be using the community package: https://synocommunity.com/packages
/path/to/ffmpeg -i "input.mkv" -map 0 -c:s copy -c:v copy -c:a ac3 -b:a 640k "output.mkv"