You can add sound to any slide of your Powerpoint presentation. This is much more required specially when you want to publish your presentation on a public forum like youtube.com.
Select your target slide. Under Insert Tab, click the Audio button on the far right. You’ll be presented with two options: either record your audio or select an audio file from your computer. This will show up a Sound icon on your slide. Click the Icon to Play and test it right away. You can also see the Audio Tools menu, click Playback from this menu and you can configure many settings like playing the sound automatically or through a click, setting up the volume, Play in background etc.