WebAudioImporter.cs

using System.Collections;
using System.IO;
using UnityEngine;
using UnityEngine.Networking;

public class WebAudioImporter : MonoBehaviour
{
    private MobileImporter mobileImporter;

    private void Start()
    {
        mobileImporter = GetComponent<MobileImporter>();
        mobileImporter.Loaded += OnAudioLoaded;

        StartCoroutine(DownloadAndLoadAudio());
    }
    
    private IEnumerator DownloadAndLoadAudio()
    {
        var req = UnityWebRequest.Get("https://file-examples.com/wp-content/uploads/2017/11/file_example_MP3_700KB.mp3");

        yield return req.SendWebRequest();

        var downloadedBytes = req.downloadHandler.data;

        var savePath = Path.Combine(Application.temporaryCachePath, "downloadedAudio.mp3");
        File.WriteAllBytes(savePath, downloadedBytes);
        
        mobileImporter.Import(savePath);
    }

    private void OnAudioLoaded(AudioClip audioClip)
    {
        Debug.Log(audioClip.length);
    }
}