Adding to "Open with" list
You need to edit Audacity's Info.plist file and add mp4 to the list of files it handles.

Using a text editor, open /Applications/Audacity.app/Contents/Info.plist (this is assuming Audacity is installed as Audacity.app, it might be slightly different).

Then, find the section for CFBundleDocumentTypes which should list the supported types like:

&lt;key&gt;CFBundleTypeExtensions&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;...&lt;/string&gt;
        &lt;string&gt;...&lt;/string&gt;
        &lt;string&gt;...&lt;/string&gt;
    &lt;/array&gt;

Then just add a new line for mp4: &lt;string&gt;mp4&lt;/string&gt; and save the file.

To get the LaunchService to pick up your change, you have to run (in Terminal):

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user

After that, Audacity should show as a Recommended Application for mp4 files.