My guess is that the driver for your sound device may or may not implement all the expected sound controls on the keyboard. That is to say that they are implemented correctly by the system drivers (speakers, headphone socket) but these are not in the core sound processes - hence you need your driver to support them.
I have a Fiio USB headphone amplifier that I run from my MBP, and neither the mute nor the volume controls work on the keyboard. I suspect volume is applied by analogue circuits, not digital ones - so this would make sense. But I'm a touch surprised my mute doesn't work; maybe that is expected to be implemented by the same thing that controls the volume. (Edit: my device didn't come with a driver, but was recognised as a USB sound device automatically).
Long story short - I would guess that if there is a specific driver for your device, that installing that - or finding the control panel - might help. If you don't have a driver file, try googling for one (and obviously, only download from sources you trust).
A multi-output device allows you to mirror audio output to multiple devices at the same time.
An aggregate device allows you to tie multiple devices together to appear as one, single, device with more I/O than any one single device has.
Aggregation has a lot of applications in music production where you might want to use more than one audio capture device at the same time from Logic Audio or GarageBand. To do this, you aggregate devices in to one new, virtual device and access the single device from within Logic.
Some examples might help illustrate the difference.
Example 1: Multi-Output Device
Let's say I wanted to play music in iTunes and have the audio go to my iMac's built-in speakers and my AppleTV via AirPlay at the same time. How would I do this? I would create a new, multi-output device. Assign both my Built-In Output and AirPlay sources to this device and then select it as my output device for audio on my iMac.
Now audio played in iTunes goes to both my iMac's speakers and my AppleTV at the same time -- it's mirrored.
Example 1: Aggregate Device
Let's say I wanted to record 4 streams of mono audio at the same time but all I have are two audio input devices that each have 2 mono streams on them (they're basically stereo capture devices). I could create an aggregate device out of the two devices (assuming their drivers support aggregation in OS X) and use this new, aggregate device in GarageBand or Logic and now, instead of seeing 2 channels of input, I'd see 4 channels of input and the devices would function as one, bigger "virtual device".
It works in a similar fashion for their outputs. They all act as independent outputs on one virtual device.
Best Answer
Try making sure that your source format (Hz) and bitrate are set to the same on both devices. My guess is that if they are different, you may have one receiving information at one rate but transmitting as another.