From the Gnome or KDE menu, open your sound configuration panel. Make sure audio boxes or headset are connected to your system and find an output level that is comfortable for you. Make sure, when your system is ALSA-compatible, that you use the appropriate panel.
If you have a microphone, try recording a sample of your own voice. Make sure the input volume is not too high, as this will result in high-pitched tones when you communicate with others, or in transfering background noise to the other party. On the command line, you might even try to use arecord and aplay for recording and playing sound.
Locate sound files on your system and try to play them.
Insert an audio CD and try to play it.
Find a chat partner and configure a VoIP program. (You might need to install one first.)
Can you listen to Internet radio?
If you have a DVD player and a movie on a DVD disk, try to play it.