Both of these pieces of equipment serve different purposes in a sound system. An audio interface is a device that converts analog audio signals into digital format for processing on a computer, and vice versa. Modern audio interfaces can often provide 48V phantom power and have built-in preamps for mic gain. It is primarily used for recording, allowing you to connect microphones, instruments, and other audio sources to your computer. We have some great recommendations for affordable audio interfaces if you're interested. On the other hand, an audio mixer, also known as a mixing console, allows you to control and adjust the audio levels of multiple audio inputs in real-time. It can mix these signals together, adjust their volume levels individually, and apply effects. While some audio mixers come with built-in audio interfaces, they are fundamentally different tools designed for different tasks in audio production.