Customizing the player is impossible.
However, you can do it differently: leave it invisible, and create your own custom controls by sending the commands directly to the player via JavaScript.
For example:
<audio id="demo" src="audio.mp3"></audio>
<div>
<button onclick="document.getElementById('demo').play()">Reproduzir o áudio</button>
<button onclick="document.getElementById('demo').pause()">Pausar o áudio</button>
<button onclick="document.getElementById('demo').volume+=0.1">Aumentar o volume</button>
<button onclick="document.getElementById('demo').volume-=0.1">Diminuir o volume</button>
</div>
* This example has been taken from the MDN website, where you can find more details on the subject: Using audio and video with HTML5 .