Probably best to use a few components..... resistor (eg. 1 kilo-ohm), basic diode, and capacitor (eg. 100 uF)..... build a peak detector. Then all you need to do with your arduino is to monitor DC voltage level. If it stays above a particular DC voltage level for X milliseconds, then audio is being generated.
At the moment, it's also possible that you could be confused between the words 'frequency' and 'AC signal'. You speak of 'detecting frequency', but you probably actually mean 'detecting presence of an AC signal'.