There are three parts to achieving the correct audio levels: setting your source volume, setting your input gain, and setting your monitoring volume.
Warning: when setting audio levels, it's easy to make mistakes that result in unexpected loud burst of volume. For example, you might set your source volume too high without your source connected to the Stream, and then, when you connect the source to the Stream, you're immediately met with a much higher volume than you expected. To avoid these accidents, always begin with all levels (gain and volume knobs) turned all the way down. When you raise levels, do so slowly.
First, you need to set the audio level on your source, e.g., turning up the volume knob on an electric guitar.
Next, turn up the large volume knob on the Stream to somewhere between 25-50%; this is so you can hear your audio when setting the input gain. Then, with your monitoring (large volume knob) set to an audible level, slowly turn up the input gain knob (smaller knobs) on the Stream until you have clear audio. Finally, adjust the large volume knob to your preferred monitoring level.
See setting audio levels and our instrument micing and connection guides for more information.