[html] Is it possible to style html5 audio tag?

I haven't found any resources on how to do that. Something as simple as changing the color of the player would be nice to have :)

Yes: you can hide the built-in browser UI (by removing the controls attribute from audio) and instead build your own interface and control the playback using Javascript (source):

<audio id="player" src="vincent.mp3"></audio>
<div> 
  <button onclick="document.getElementById('player').play()">Play</button> 
  <button onclick="document.getElementById('player').pause()">Pause</button> 
  <button onclick="document.getElementById('player').volume += 0.1">Vol +</button> 
  <button onclick="document.getElementById('player').volume -= 0.1">Vol -</button> 
</div>

You can then style the elements however you wish using CSS.

MDN HTMLAudioElement API reference

If you want to style the browsers standard music player in the CSS:

audio {
    enter code here;
}

The appearance of the tag is browser-dependent, but you can hide it, build your own interface and control the playback using Javascript.

Yes, it's possible, from @Fábio Zangirolami answer

audio::-webkit-media-controls-panel, video::-webkit-media-controls-panel {
                background-color: red;
}

To change just the colour of the player, simply address the audio tag in your css file, for instance on one of my sites the player became invisible (white on white) so I added:

audio {
    background-color: #95B9C7;
}

This changed the player to light blue.

Missing the most important one IMO the container for the controls ::-webkit-media-controls-enclosure:

&::-webkit-media-controls-enclosure {
    border-radius: 5px;
    background-color: green;
}

<audio>

audio::-webkit-media-controls-panel

audio::-webkit-media-controls-mute-button

audio::-webkit-media-controls-play-button

audio::-webkit-media-controls-timeline-container

audio::-webkit-media-controls-current-time-display

audio::-webkit-media-controls-time-remaining-display

audio::-webkit-media-controls-timeline

audio::-webkit-media-controls-volume-slider-container

audio::-webkit-media-controls-volume-slider

audio::-webkit-media-controls-seek-back-button

audio::-webkit-media-controls-seek-forward-button

audio::-webkit-media-controls-fullscreen-button

audio::-webkit-media-controls-rewind-button

audio::-webkit-media-controls-return-to-realtime-button

audio::-webkit-media-controls-toggle-closed-captions-button

REFERENCE: https://chromium.googlesource.com/chromium/blink/+/72fef91ac1ef679207f51def8133b336a6f6588f/Source/core/css/mediaControls.css?autodive=0%2F%2F%2F

Ken had it right as well.

a css tag:

audio {

}

will get you some results. seems it doesnt want the player any height above or below 25px but the width can be shortened or lengthened to an extent.

this was good enough for me; see this example (warning, audio plays automatically): www.thenewyorkerdeliinc.com

You have to create your own player that interfaces with the HTML5 audio element. This tutorial will help http://alexkatz.me/html5-audio/building-a-custom-html5-audio-player-with-javascript/

some color tunings

audio {
    filter: sepia(20%) saturate(70%) grayscale(1) contrast(99%) invert(12%);
    width: 200px;
    height: 25px;
}