Example code showing how to mute Tracks in a Timeline asset.

MuteTimelineTracksScript.cs

/// <summary>
/// Instructions 
/// - Create a Timeline asset. Add 3 tracks to it.
/// - Create empty GameObject in scene. Attach this script to the GameObject. Then attach a PlayableDirector component to it. 
/// Drag and drop the Timeline asset you created into the Playable field of the PlayableDirector.
/// - Hit Play. The 1, 2 and 3 keys on your keyboard mute the first, second and third tracks respectively.
/// </summary>

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Timeline;
using UnityEngine.Playables;

public class MuteTimelineTracksScript : MonoBehaviour {

	public PlayableDirector playableDirector; // Add reference to attached PlayableDirector component from Inspector
	private TimelineAsset someTimelineAsset; 

	// Use this for initialization
	void Start () {

		someTimelineAsset = (TimelineAsset)playableDirector.playableAsset;

	}
	
	// Update is called once per frame
	void Update () {
		
		if(Input.GetKeyDown(KeyCode.Alpha1)) MuteUnmuteTrack(0);
		else if(Input.GetKeyDown(KeyCode.Alpha2)) MuteUnmuteTrack(1);
		else if(Input.GetKeyDown(KeyCode.Alpha3)) MuteUnmuteTrack(2);

	}

	void MuteUnmuteTrack(int trackIndex)
	{
		// Get track from TimelineAsset
		TrackAsset someTimelineTrackAsset = someTimelineAsset.GetOutputTrack(trackIndex);
		
		// Change TimelineAsset's muted property value
		someTimelineTrackAsset.muted = !someTimelineTrackAsset.muted;
		
		double t = playableDirector.time; // Store elapsed time
		playableDirector.RebuildGraph(); // Rebuild graph
		playableDirector.time = t; // Restore elapsed time
	}
}