AudioEchoFilter.wetMix  

// Mixes both Echo generated sound and the original audio source
// if you set the wetMix to 0 you will not have Echo sounds.

@script RequireComponent(AudioSource)
@script RequireComponent(AudioEchoFilter)

function Start() {
    GetComponent(AudioEchoFilter).wetMix = 1.0;
    GetComponent(AudioEchoFilter).dryMix = 1.0;
}

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
[RequireComponent(typeof(AudioEchoFilter))]
public class example : MonoBehaviour {
    void Start() {
        GetComponent<AudioEchoFilter>().wetMix = 1.0F;
        GetComponent<AudioEchoFilter>().dryMix = 1.0F;
    }
}

import UnityEngine
import System.Collections

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

    def Start():
        GetComponent[of AudioEchoFilter]().wetMix = 1.0F
        GetComponent[of AudioEchoFilter]().dryMix = 1.0F