If you leave the mute unconnected, it will mute both outputs (>= 70 dB of attenuation). If you ground it, both outputs will be on.
In an RF prone environment, it would be wise to bypass the mute pin to ground with a ceramic capacitor.
If your goal is to control this via an audio source, you will need to add a comparator circuit to the design. This would allow you to use a pot to set the mute level.
Keep in mind that the AN7511 is an obsolete chip.
- Glenn W9IQ