Check for extension problem:
Reboot your Mac without extensions: OS X Yosemite: Start up in safe mode
If you don't hear the boot chime sound, just press the ⇧ shift 3 seconds after the power on button. Keep it depressed until you see the dark grey progression bar showing the extensions, cache and kernel reorganization in progress.
Check if the sound is OK.
If yes, start the hunt for the bad extension.
If not, you have an hardware problem. Bring your Mac to Apple.
Check for extension, cache or kernel corruption:
Verify if the audio kernel extensions are actually loaded:
kextstat | egrep 'Index|Audio'
You should get an output very similar (but the hex address field) to this:
$ kextstat | egrep 'Index|Audio'
Index Refs Address Size Wired Name (Version) <Linked Against>
95 4 0xffffff7f81603000 0x30000 0x30000 com.apple.iokit.IOAudioFamily (203.3) <94 5 4 3 1>
196 0 0xffffff7f82c24000 0x5000 0x5000 com.apple.driver.AudioAUUC (1.70) <95 76 12 11 7 5 4 3 1>
$
If you don't have these 2 extensions loaded, you will take advantage of this default to load them in a mode where you will see what is going wrong (i.e. what is usually hidden behind the nice spinning beach balloon®).
To manually reload a missing extension and to take advantage of this manual operation to better understand the origin of the bad, here is how to proceed:
/usr/bin/sudo kextutil -t -v 6 -b com.apple.driver.AudioAUUC
which loads the kernel extension com.apple.driver.AudioAUUC
-b identify it with its bundle name,
-t print diagnostics,
-v 6 will be as verbose as possible about every steps of this process.
At the end of this verbose kernel extension loading, check that the kernel has loaded this extension:
kextstat -b com.apple.driver.AudioAUUC
Best Answer
System Preferences builds a preferred list for your audio outputs and inputs based on which ones you pick over others.
To work around this you will need to manually select the audio output you would rather use once when the undesired one is available and again when it isn't. This method will work in most (but not all) cases.