This guide has been updated with details on how to record line-level audio through the USB audio adapter! Just tested it on Jessie Lite and it works great!!
The Raspberry Pi has an on-board audio jack, which is super handy for all kinds of sound effects and speech, just plug and go! However, for when you want better audio for music playback, a USB audio card can greatly improve the sound quality and volume, this tutorial will show you how.