Final answer:
Mixing down multiple audio channels in music production.
Step-by-step explanation:
In music production, mixing involves combining multiple audio tracks together to create a cohesive and balanced final product. In this case, the student is referring to a song that had four separate channels of strings, and the task was to mix them down to a single mono track. Mixing down refers to the process of reducing multiple audio channels to a single channel, often used to create a mono or stereo mix.