Internal Speakers Disappear

After a reboot, my audio output choices include the internal speakers, and they work well with the first audio/video link. But the fist time I go from my browser to maybe itunes, and then back to the browser, the audio plays, but there is no sound. A quick check of the System Preferences shows the only output choice as earphones, which are not, and never have been connected on this MacBook Pro. It is running OSX 10.9.2. Is there a simple fix for this problem?