In order to embed an audio element on a Wix Blog post (like this one) you will have to host your audio source on a compatible platform online. (SoundCloud is not a compatible source.)
Hosting audio on Publitio
The following audio file is hosted on the Publit.io platform where they offer a free plan and a premium for less than $10 per month.
Sample music audio file:
Generic HTML code:
[Warning: This method allows users to hover and right-click audio player to download file.]
<audio controls> <source src="https://media.totallycodeable.com/file/file_example_MP3_700KB-G.mp3" type="audio/mpeg"> Your browser does not support the audio element. </audio>
Notice that the blue URL is the source of the audio that is being hosted on the Publit.io platform.
Within the Wix Blog editor click on the plus sign to add HTML code. Paste the sample code above and simply replace the sample audio source URL with your own.