Here's a quick lesson on audio signals: Traditionally, audio signals are analog, which is good because speakers require an analog signal to make sound. However, every source of data from a device is digital. This digital signal is converted in something called a DAC (digital to analog converter). When it comes to sound signals, usually a headphone jack is the DAC (the digital sound is converted to analog in the headphone port and an analog signal goes up to your headphones). Now, The Nexus 7's internal DAC is pretty weak and sound quality would suffer when played by large car speakers (I'm no audiophile, but I just wanted to do this right). However, the makers of this tablet allowed for the digital audio signal to be an output of the micro-USB port. This "USB Audio" allows the audio data to travel through digital devices, such as a USB hub, as well as being converted by something other than the headphone jack.