Use a sound recording site on your Chromebook. Vocaroo is easy because no account is required and you can download the file afterward.
It will download to the Files app on your Chromebook as a .mp3 or .wav file
Determine which type of recorder you have
Use the USB port on the sound recorder and plug it into your computer or chrome book. It should show up as "USB Drive" in your Files app.
Select the old mp3 files on it and hit the trash can icon.
Go into your downloads folder and drag any new mp3 files from there onto the USB Drive.
Hit the Eject symbol (triangle above a line) to eject the USB Drive .
The sound should now be on the Digital Sound Unit.
Look below under "Model B Sound Recorders"
Very often the wires are not soldered onto the digital sound unit. Or it simply doesn't work. Then what you need to do it program a servo to hit the button with a miniature hammer to turn it on.
This is an example:
And here is a video of it in action: