Agueda, when you click either the Count Down or Count Up button in the Clock tool, does the middle field — the “play the following sound” field shown with the number 2, below — offer any options other than No Sound when you click that field?
When in that
window, if no sounds are available in that drop-down window, click on the “…”
button beside that drop-down to browse to the folder on the computer where the
desired sound files are located. Browse into the folder (you will not see
any files in the folder) and click “Select Folder”. You will see the
sounds in that folder in the drop-down menu after choosing that folder.