No Sound from Internal Speakers?

[PhilPhletcher]

On occasion, sound won’t play from the internal speakers on my iMac. I have made sure that nothing is plugged in (nothing in the headphone jack, no external displays, etc), made sure it wasn’t muted, and set the system output to Internal Speakers. Any ideas?
Also, I found in the sound options a something called “System Sound Device.” It’s not on my other mac, and I have no idea what it is or how it got there.

Edit: I just realized that this only happens when I have GarageBand open. Most likely to prevent feedback. But I’m still confused about the System Sound Device thing.
This is what it looks like in System Preferences:

[bowline]

I’m not sure exactly what’s going on but Garageband may have created a virtual audio device for output/recording which siphoned the audio from your internal speakers. (Or you downloaded another app which implemented a sound driver that took charge.) You might get a better idea of what’s happening by launching the built-in Audio Midi Setup app top see what’s going on in a more detailed view. Here’s mine: