Exactly as the title says.
I do not have surround sound and I would like to make the movie a little smaller by removing the extra (on this movie 6) audio tracks that I would never hear anyway. I prefer not to downsample audio, but if I must then I will.
I know the free audio tool audacity can separate tracks but it cannot merge them back to ac3, as it only exports .wav and also it can convert into other formats.
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You can't just remmux only the left and right channels. Most of the dialog is usually on the center channel. So the audio has to be mixed and re-encoded. Some ffmpeg command lines can be found here:
Next to ffmpeg, eac3to can also convert 5.1 AC3 to AC3 stereo. You may wanna use UsEac3to as a GUI.
If you mean the phrase "downsampling" more generally as also being downmixed into fewer channels, take notice of what will happen during AC3 5.1 to AC3 2 channel.
In short, 5.1 AC3 will be decoded into six mono pcm streams. These will be remixed to 2 channel pcm, which will then be encoded back into AC3. You can imagine this process is not lossless.
I don't know what equipment you use, but maybe your playback device can do the down-mixing for you. In case you have a surround amp with just 2 speakers connected, the amp could probably do it for you.
If any device in your playback chain is not able to handle surround sound in any way, then indeed you'll need to convert the audiosource to stereo.
[EDIT] Sorry jagabo, you beat me to it by a few minutes...
Thank you all for the replies. I really appreciate it. I will also check out the mentioned tools and see what works best for me.