Just wanted to add here, 3 years late to the discussion. : )
I am running a mid 2010 Mac Pro / Yosemite with Bose companion 5 speakers.
Note that my mid-2010 Mac Pro has USB 2.0 ports, and the Bose companion 5 speakers require as such.
I was never able to get these speakers to appear in the sound dialog under system preferences.
However, If you go to your Utilities folder (under your 'Go' menu item from the desktop), and launch the Audio MIDI set up application, I saw the Bose speakers there in the left hand list.
Select the Bose speakers from the left-hand list.
In the right-hand area of the same dialog, you should see some information about the Bose set up.
Make sure that the 'mute' check box next to the master volume control is unchecked.
Additionally and at the bottom of that list are three buttons: a '+' a '-' and an icon that looks like a gear, or cog. Click on that cog.
From the resulting drop down dialog, select, "use this device for sound output."
That should do it.
As an aside, you can see if your Mac Pro USB port is recognizing the Bose speakers in the first place by doing this:
From the finder, click on the apple icon in the upper left
Select 'About this Mac'
Click, 'System Report...'
From the new dialog that appears, select, 'USB' from the left hand list. The list is displayed alphabetically. 'USB' is under the Hardware header.
All of your USB busses are listed. If your Mac is recognizing the Bose speakers, it should be listed as so under one of the 'USB Bus' listings. If it is not, you may need to restart your machine or unplug the power from the speakers and then plug back in. Or both. Or perhaps you have a bad USB cable.
The Bose Companion 5 speakers are GREAT sounding. They are terrible however with getting them set up. Additionally, they just stop working for no reason on an intermittent basis and this always sending me surfing again on the interwebs.