Thanks, Tim. Yes, I've been using Audacity pretty much as Justaguy's older posting describes, except that I generally just export the tracks directly to MP3 rather than going through the intermediary "quickmix" step. Worked great until last night. Somehow, I must have unchecked that setting under Preferences, assuming that's the problem. And I'll bet it is.
The other thing I thought of doing to reset the application was to go into the Audacity folder and see if there's an INI file for the app, then delete it. These files are commonly used to hold the settings between instances of running the application, though some apps do it differently, such as through Windows Registry settings. When you delete it, the app just recreates it the next time it's launched, using the default settings. A handy trick for situations like this.
Thanks also for the tip on background clips. Yes, I found that thread and used it to get a couple. Plan to poke around and get a few more this weekend.
Appreciate your help. Hope you're enjoying the Florida weather. A squall blew through here this morning and dumped an inch and a half of snow in about an hour....