Im new in unity and c# - i tried to make a image fill amount decrease.

 while (goAudioSource.isPlaying){
                  tempo.GetComponent<Image>().fillAmount -= 1.0f/waittime * Time.deltaTime;                
                  
      if(tempo.GetComponent<Image>().fillAmount == 0.0f){
                      goAudioSource.Stop ();
      }

here you are decreasing to 0 in single Update cycle. what you want is animating effect.
for that change while with if so that in every frame it will decrease a little.

It does not work right . It decreases only once. Here is my complete code with the if .

public class microfone : MonoBehaviour {

     public int contador;
     public int cont;
     public string bot;
     public float waittime = 1.0f;
        private bool micConnected = false;  
     Text  tocarbotao;
     private int minFreq;  
     private int maxFreq;  
     

void Start()
{
contador = PlayerPrefs.GetInt ("verdade"); //Contador inicialmente recebe o valor de numero de perguntas cont=contador; // O cont vai ser igual ao contador PlayerPrefs.SetInt("contador",cont);

     tempo.GetComponent<Image>().fillAmount = 0;
       
         if(Microphone.devices.Length <= 0)  
         {  
             Debug.LogWarning("Sem Microfone!");  
         }  
         else   
         {  
             micConnected = true;  
             Microphone.GetDeviceCaps(null, out minFreq, out maxFreq);  

             if(minFreq == 0 && maxFreq == 0)  
             {  
                 maxFreq = 44100;  
             }  

             goAudioSource = this.GetComponent<AudioSource>();  
             }  
             
     }  

     public void Tocar()
  {
     tocarbotao = gameObject.GetComponent<Text> ();
     bot = tocarbotao.text;
             
     if (cont > 0){ 
         if (bot == "GRAVAR") {    
             
             if (micConnected) {  
                 
                 if (!Microphone.IsRecording (null)) {  
                     goAudioSource.clip = Microphone.Start (null, true, 20, maxFreq);  
                     gameObject.GetComponent<Text> ().text = "ANALISAR";
                     }  
             } 
         } else { 
             gameObject.GetComponent<Text> ().text = "GRAVAR"; 
             Microphone.End (null);  
             goAudioSource.Play (); 
             
             if(goAudioSource.isPlaying){
                 tempo.GetComponent<Image>().fillAmount -= 1.0f/waittime * Time.deltaTime;                
                 
             if(tempo.GetComponent<Image>().fillAmount == 0.0f){
                     goAudioSource.Stop ();
                     gameObject.GetComponent<Text> ().text = "GRAVAR";
                 }
             }
         cont=cont-1;    
         }  
         if (cont == 0){
             gameObject.GetComponent<Text> ().text = "FIM";
     }