The Raspbian Wheezy installation on your Pi uses a software sound architecture called the Advanced Linux Sound Architecture (ALSA). ALSA drivers allow dynamic sensing of audio devices as the system boots, or when a device is plugged in—if it is USB-based. The Pi hardware supports two sound output methods directly on the motherboard but does not have the microphone recording capability. This audio output capability is actually built into the Broadcom SOC. The inbuilt sound output channels supported are stereo headphone and line output on a 3.5 mm jack with combined composite video and digital audio over the HDMI connector.

The line output audio signal is generated using a Pulse Width Modulation (PWM) signal and is limited in audio bandwidth and power output. We recommend that you do not use low-impedance headphones (less than 16 Ohm) as the distortion and noise on the channels tends to increase. There are many high-impedance headphones available...