AudioHighPassFilter.cutoffFrequency  

// Moves the cuttoutFrequency from 10 to 22000 following a Sinus function
// Attach this to an audio source with a HighPassFilter to listen it working.

@script RequireComponent(AudioSource)
@script RequireComponent(AudioHighPassFilter)

function Update() {
    GetComponent(AudioHighPassFilter).cutoffFrequency = (Mathf.Sin(Time.time)*11010 + 11000);
}

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
[RequireComponent(typeof(AudioHighPassFilter))]
public class example : MonoBehaviour {
    void Update() {
        GetComponent<AudioHighPassFilter>().cutoffFrequency = Mathf.Sin(Time.time) * 11010 + 11000;
    }
}

import UnityEngine
import System.Collections

[RequireComponent(AudioSource)]
[RequireComponent(AudioHighPassFilter)]
class example(MonoBehaviour):

    def Update():
        GetComponent[of AudioHighPassFilter]().cutoffFrequency = ((Mathf.Sin(Time.time) * 11010) + 11000)