Thanks! For some reason I'm not seeing all new messages when I click on "Show unread posts since last visit." I guess I'll have to click on "Show new replies to your posts," too.
Before I read your message I managed to fix it (I think). It occurred to me that I'd seen messages on the forum about people's webcams interfering with their sound, so I did a search and found a reference to http://forum.vectorlinux.com/index.php?topic=4888.msg32619#msg32619
. I tried the fix and when I rebooted and started Opera, lo and behold it played the sound! I figured the situations were basically the same.
Now I'll check your suggested links and see if you described the same fix.