Keynote help

If the file is placed on a slide;

click the sound file to select, then go to Inspector > Format > Audio and drag the file to the desktop, Edit the file then delete the initial file on the slide and replace with the edited file.

If the file is placed in Soundtrack;

go to File > Advanced > Change File Type > select Package, then Save

Right click the Keynote file and select - Show Package Contents

in the Finder window that opens, click on Data then drag the sound file to the desktop

Edit the sound file and save ensuring you use exactly the same name used in the original

In Keynote, select the sound file in SoundTrack then press the backspace key to remove it

Add the edited sound file to Keynote and save

I did mention that, but just to be clear:

from a Keynote package filetype,

Right click the Keynote file and select - Show Package Contents

in the Finder window that opens, then click on the Data folder

Keynote saves project files as a "Single" file by default, but as I explained you can change any existing Keynote files:

File > Advanced > Change File Type > select Package, then Save