Convert DTS audio to Opus/AAC mkvtoolnix

[UserName351]

Hello,

I have 2 sets of mkv files, one is HEVC and the other is 264.
I'd like to take the HEVC ones (remove useless subtitles) but keep the rest, and take the 264 ones and only keep the chapters, 2 subtitles, and 1 audio.
Doing this on the mkvtoolnix GUI, then take the command line and adapt the lines for all episodes. (I was initially thinking of extracting them then adding them as separate files, but adding both HEVC and 264 in the GUI and only keep what I want seems better)

However, the audio from the 264 files is around 255 MB, more than the HEVC files... So I'd like to convert the new audio which is DTS to Opus to save some disk space. Can I do that using mkvtoolnix ? (Like an extra command option ?) Or do I have to extract all the wanted audios and convert them using something else ?
If I have to use something else, is Ffmpeg good for that?

Thx for any imput

[ProWo]

Can be done by clever Ffmpeg-GUI, convert audio section. After conversion, you can mux the new created audiostream with your hevc movie, selecting the streams you want, in the multiplex section to mkv/mp4 or other container.

[UserName351]

Can be done by clever Ffmpeg-GUI, convert audio section. After conversion, you can mux the new created audiostream with your hevc movie, selecting the streams you want, in the multiplex section to mkv/mp4 or other container.

Thank you, I'll keep that one, but it seems to be good when dealing with just a few files, I'd like to be able to batch mux

[UserName351]

Well, I used this command in a batch:

for %%a in ("*.mkv") do ffmpeg.exe -i "%%a" -map 0 -c:v copy -c:s copy -c:a libopus -b:a 192k "newfile\%%~na.mkv" & move "%%a" "oldfile\%%a"

The overall file size did change, MediaInfo says the audios are now Opus, howerver MediaInfo says still has the same old audio bitrate and same size in MB. So I guess something went wrong somewhere.
Also I wanted to use --vbr on, but I found it is on by default, but if I wanted to have it in the command, where should I add that option?

Edit: Just tried with StaxRip, set it to passthrough on video, an convert audio to Opus 192kbps, but it seems that the audio is a little bit cut off at the start and abruptly starts (i don't think there any delay)