I am only familiar with All Metal audible threshold machines, so I want to talk a little about how they react. Rudy is correct, you want your threshold set to a constant low hum that is just audible over background noise. Setting it too high will help you miss targets.
In all metal, like when you are looking for gold nuggets with a Tesoro Lobo, or Fisher Goldbug II, you are listening for "Positive Breaks," or a normal "Zip-Zip" signal, and "Negative Breaks," which are silent breaks in the theshold. In Rudy's example, Silent breaks or "nulls" mean Ferrous metal, in my example, they are DEEP targets. I am not sure why a discriminating machine would use an audible threshold. Maybe Rudy has the answer..