5.1 audio refers to a type of surround sound setup that consists of six channels: five full-range channels (left, center, right, left rear, and right rear) and one low-frequency effects (LFE) channel for the subwoofer. This configuration provides a more realistic and immersive audio experience compared to traditional stereo sound. The "5.1" designation comes from the five full-frequency channels (20 Hz to 20 kHz) and the one subwoofer channel (typically 20 Hz to 120 Hz).
Here are some popular sources for downloading 5.1 audio content: 5.1 audio download