Currently in the latest stable 3.x version, there is only insert video button, no audio button. I'd like to upload mp3 file and can be rendered as an HTML 5 audio player in the browser, or even other player instead, which can prevent user download the file.

Well, the button only creates the html code
<div class="ckeditor-html5-video" style="text-align: center;">
<video autoplay="autoplay" controls="controls" src="/survey/upload/surveys/849484/files/01-Haruki%20Murakami%20_%200101.mp3"> </video>
</div>
As you see I selected a mp3 file
Just change "video" to "audio"
<div class="ckeditor-html5-video" style="text-align: center;">
<audio autoplay="autoplay" controls="controls" src="/survey/upload/surveys/849484/files/01-Haruki%20Murakami%20_%200101.mp3"> </audio>
</div>

No need for more buttons.
To be honest: I never use them, but enter the code directly (more flexible, media file can be anywhere on the server)

Joffm