Impossible 2 mux mp43 and AC-3 files into AVI container; DVD-from-USB issue

[Pompilius]

Hello everybody!

Long story short: last month, I've become the owner of a vintage LG HomeCinema System - a DVD player (with USB port) + a 5.1 system (loudspeakers + subwoofer), in mint condition (meaning: in the original unopened box). The system was made in April 2006.
Now, the DVD being a deprecated thingy, I've intended to watch videos via USB port, which I succeed. But the annoying thing is that I'm watching only videos transcoded from mp4 or mkv with stereo audio (while some of the originals have 5.1 or 6.1 or even 7.1 audio). For transcoding I'm using the latest Wondershare Video Converter Ultimate, because Handbrake discontinued the support for avi, divx, xvid etc. and the 0.9 version lacks of features.
My problem (help needed!): I want to watch movies with 5.1 audio. The manual of this DVD says that playable codec format are "DIVX3.xx, DIVX4.xx, DIVX5.xx, XVID, MP43 and 3IVX". I've tried all of them (all with 2.0 audio, of course) and the highest video quality is provided by MP43, the Microsoft-MPEG4 V3 files, which, at only 1500 kb/ s, look really amazing on a 32" ULED Toshiba TV. The other (DivX, for example) are heavy pixelated, at 1800.....2500 kb/ s, even though the source was a crystal-clear HEVC HD video. So I made two files from the mkv: a mp43 video file, @ 768x576 (maximum admitted by named LG being 800x600) and an audio file, AC-3 5.1 @ 320 kbps.
I've tried to mux the two files into an AVI container, but:
· AVI-Mux GUI v1.7.8.3 loads the 2 files, but is showing only the option to mux/ convert into mkv container (????????? - why???????);
· VirtualDub v1.10.4 says "Couldn't locate decompressor for format 'mp43' (Microsoft High-Speed MPEG4 V3). VirtualDub requires a Video for Windows (VFW) compatible codec to decompress video. DirectShow codecs, such as those used by Windows Media Player, are not suitable", though i have the x264vfw codec (x264vfw_full_44_2851bm_44825) installed;
· Avidemux v2.7 (64 bits) opens the video file, then (trying to open the audio file) says "Info: Cannot find a demuxer for -whatever-.ac3"...

All I want is: to make a file, with HQ video (well, the "HQ" allowed by a maximum 800x600, mind you) and 5.1, which can be played via USB. Any thoughts?

And the other problem: why I can't play the decrypted DVD or the DVD files from my USB? I've tried like this:
1. USB\MOVIENAME (folder)\VIDEO_TS (folder with the specific files) - nothing;
2. USB\VIDEO_TS (folder with the specific files) - nothing;
3. USB\just the files from the "VIDEO_TS" folder - nothing.
Extremely strange, because not only it doesn't show the folder or the files, it shows "USB 🚫" (LOL).

Very impatiently, I'm thanking you in advance.

[poisondeathray]

You should be able to use ffdshow , and enable mp43 in the vfw (yellow) interface for it to work in vdub



or you can use ffmpeg

eg.

ffmpeg -i video.avi -i audio.avi -c:v copy -c:a copy output.avi

You might have to set the aspect flag, depending what the source is

Or use ffmpeg to do it all including scaling, encoding direct from source file -c:v msmpeg4v3 will be the codec