Timelines built-in audio track requires some internal components to work. It calls AudioClipPlayable.Seek explicitly, which you can replicate using the following modifications to the script provided.

using UnityEngine;
using UnityEngine.Audio;
using UnityEngine.Playables;
using UnityEngine.Timeline;

[CreateAssetMenu]
public class CustomAudioPlayableAsset : PlayableAsset, ITimelineClipAsset
{
public class SeekBehaviour : PlayableBehaviour
{
public AudioClipPlayable clipPlayable;

public override void OnBehaviourPlay(Playable playable, FrameData info)
{
clipPlayable.Seek(playable.GetTime(), 0);
}

public override void OnBehaviourPause(Playable playable, FrameData info)
{
clipPlayable.Pause();
}
}


public AudioClip clip;
public bool propagate;

public ClipCaps clipCaps => ClipCaps.All;

public override Playable CreatePlayable(PlayableGraph graph, GameObject owner)
{
var acp = AudioClipPlayable.Create(graph, clip, looping: false);
var sp = ScriptPlayable<SeekBehaviour>.Create(graph, 1);

sp.ConnectInput(0, acp, 0);
sp.SetInputWeight(0, 1);

sp.GetBehaviour().clipPlayable = acp;

return sp;

}
}