Alert sounds no longer playing

I recently turned off all alerts for iChat and Skype because there were just too many sounds for people signing in and out, etc. However, I've also noticed that now I don't hear any sounds when I move things to Trash, move files around in folders, and get new mail in Thunderbird. I kind of liked that. Also, when I go into System Preferences, I can't hear a sample of the alerts when I scroll over them or click on them, which I used to be able to do. It seems that all alerts are silent. What happened and how can I fix it?

Thanks!
Kasia

Edit: I just had to restart my computer. I had some software updates installed but hadn't restarted the computer yet. The restart helped, and all is good.