I downloaded a 3.9MB song (.mp3) and converted it to (.ogg) with this command
ffmpeg -i desktop-login.mp3 desktop-login.ogg
The file size of the converted file was 23.1MB then I copied it to
/usr/share/sounds/ubuntu/stereoand i opened it to check if it got copied (replaced with the original) properly, and it was.
But when i start the computer the login sound doesn't play.