Before we replace the achievement sound for Retroarch, you must change some things with it if they have not been changed.

1. Make sure your audio file is a .ogg audio file.

2. Change the name of your audio file to "unlock".

Your properties menu should have the "Type of file" category changed to "OGG file (.ogg)" and your file name changed to "unlock".



This is not required but it is a recccomendation, keep your achievement files only a couple seconds long (2-4 seconds specifically), since longer achievement sounds can be more obnoxious in my personal opinion.

Now to find the original directory of your achievement sound file.

For most people it would be located in:

C:\Program Files (x86)\Steam\steamapps\common\RetroArch\assets\sounds

But if it's not in that exact directory then you've likely installed Steam or your Steam games in a different hard drive, if you have, locate Retroarch there and once you do, click on it locate "\assets\sounds" folder, since that would be where your original achievement sound file would be located.

Now to replace the original achievement sound file, once you've found your achievement sound file, drag the new one over into the same directory.




You should see a pop up like this:



Select "Replace the file in the destination".


Now you are done with replacing your achievement sound file in Retroarch.

Credit: https://retroachievements.org/viewtopic.php?t=11592#:~:text=All%20you%20have%20to%20do,folder%20in%20your%20RetroArch%20folder.

(Original Retroachievements post that helped me with replacing the sound file)