Importan information: The file name of the sound file should only consist of a-z, 0-9 and _ .  So no special characters like åäö, ü or any other.

To add a new soundfile, you click Tools > Sounds in the left menu.

Click on the upload sign in the bottom right corner.

Click on upload and choose the file you want to upload.

Select the soundfile you want to upload and click open.

Now you should see your newly uploaded soundfile.