Different sound speeds

[KAF2000]

This is probably a basic question, but I have Bud Spencer and Asterix movies that have only been released in German/Italian on Blu Ray. The only English language releases are on DVD. The cut is the same. If I rip the DVD audio and add it as a new channel using the Video-Nix tool, it is a different length. What is the best way of stretching/compressing audio to fit a time, without any weird audio artifacts, using easily available, cheap/free software (Audacity gave it a strange echo) please?

[WXM]

Eac3to is a very tiny command line tool for all kinds of audio conversion for video, such as PAL<->NTSC slowdown/speedup.  The downside is that you need to know what you want to do pretty specifically, and the input/output formats can be a pain IIRC (such as it not liking FLAC [EDIT: apparently FLAC is fine to use; I don't fully recall what...]) AND if you're not used to CLI usage you might want to forget it.

Have a look as see how it feels for you. The upside is that, again, it's very small, isn't some big monster you need to install, so if you don't like it there aren't all kinds of tendrils to pull out of your system. (There's a GUI version, but I ended up going back to the command line version, I don't remember why, been a while since...)
https://www.google.com/search?&q=eac3to

[velocity]

Latest eac3to can be found here: https://www.rationalqm.us/eac3to/
You can use options like -25.000 -changeTo23.976 to convert 25->23.976 and vice-versa.

Be aware that eac3to does not perform pitch correction, so it will change both speed and pitch. Most of the time this is what you want, but sometimes the source track has correct pitch (meaning the distributor applied pitch correction on their side). In those cases you would need to apply pitch correction yourself, but doubling that process usually sounds pretty bad.

There are other options for this, e.g. an audio editor, or in also command line, sox (where the syntax is decidedly less user friendly). SoX should theoretically be higher quality.

@WXM I never had issues with handling FLAC with eac3to, that might have been in an older version.

[BusterD]

Yeah, unless you care more about the German/Italian audio, it's better to change the video using eac3to, since that's a lossless change, and you don't need to worry about pitch or anything. A command like the following should work:
eac3to C:\example\file.h264 C:\example\file2.h264 -25.000 -changeto23.976
The resulting video might not be fully Blu-ray compliant, though, slowing down 25fps video from a 1080i50 Blu-ray container stream to 23.976 resulted in either choppy playback or no playback depending on the player when I tried it after burning the file to a BD-R, but I imagine playback using a PC or other file-based playback would be fine.