Hi. I have a bunch of movies that were saved with VCD and SVCD in mind
The way you express yourself is it possible that these are still in avi format but with vcd/svcd type resolutions?
If yes the virtualdub will let you open the avi, compress the audio only and save the new avi without changing the video compression.
What you need to do under the video menu is select Direct stream copy instead of full processing mode.
If they are vcd or svcd then you need to demux the 2 streams, then compress the audio and then remux the video and new audio. This will not change the video and you do not need to recompress the video .
Since you have tmpgenc you just need to change what you were doing a bit.
I use tmpgenc v2.5.13… in the file menu, select Mpeg Tools Simple demultiplex and choose the type of file you have and the inputs. This will create 2 files containing the video and the audio. Once you have the outputs, use the audio only and re-encode it with any audio processing program.
List of Audio encoders
List of audio editors
After you are done just choose Simple Multiplex and the new audio and the original video split in demultiplex step.
You will also note that tmpgenc has a simple cut and merge/join option for alike clips before or after you have corrected the audio.