What does the output waveform from audio source look like?
A waveform for audio can be seen in two ways, the Frequency Domain and the Time Domain. Any common audio application tends to have a visualizer that can show either.



This image shows a simplified view of how different frequencies meld into each other and change the waveform.


(source: electronics-tutorials.ws)  

To cut off part of a waveform, you would use a band pass filter, which only allows a certain section of frequencies (bands) to pass through. A common consumer example of these are EQs, audio equalizers. They can be simple Bass/Mid/Treble or larger multi-frequency ones.





They allow you to Cut Off or Boost/Amplify certain frequencies.

Now more to your point, you want to light leds with only frequencies below 700Hz. Using a bandpass filter, a lowpass essentially, you can cut off most of the frequencies present, and the rest will light your leds. They will flash mostly based on the amplitude of the waveform in those frequencies, using them like a turn on signal. This is called a Color Organ.



This circuit has a pre-amp, and three bands, using a lowpass, a midpass, and a highpass Resistor Capacitor circuit, connected to a transistor and led for each. This covers Treble, Mids, and Bass. They can be done independently, you only need one. You can use a more specific, custom bandpass RC filter (google for calculators) to filter close to the frequency you want.

There is also the LM3914/5/6, which is a linear, logarithmic, and dedicated VU-meter IC. You can use a bandpass before the input to only measure the strength of those frequencies you choose. 

Newer options involve an IC (like the MSGEQ07) that takes an analog signal, digitizes with an ADC, and spits out frequency band information digitally, for use with a microcontroller. This of course, is a bit more complicated/involved, but makes for a great project.