v.1. To combine all of the audio components of some recording into a single final soundtrack or mix: We recorded the last guitar part toward the end of the song, and now we can mix the song down. It can be difficult to mix down 24 tracks.
2. To reduce the volume of some component of an electrical or audio signal relative to other components: The drums sounded too loud in the recording studio, so we mixed them down. The sound engineer mixed down the vocals.