For an example of a blog with an invisible music player, go to Invisible background music player with your speakers turned on.

I don't recommend this, but there are bloggers who want it, so I will describe how first and explain why I advice against it later.

The very first thing you have to do is to upload your music file to a online file host which will put your music file online and not those that let you upload the music file and require anyone to listen to the music to click on a link to download the music file.

Once you have uploaded the music file to the web, get its URL.

Sign into blogger.com (Dashboard) and you will see a list of blogs you have created. Of course if you only have created one blog, you will see only one. Whatever it is, click LAYOUT and in any section, I recommend the footer, click "Add a Page Element", and in the "pop-up", select HTML/Javascript, then add this script into the HTML/Javascript box:

You will of course have to substitute URL of music with the URL of your own music which had been uploaded to the web.

The attribute loop="true" play continuously. If you want it to play only once, you will have to change it to loop="false".

[UPDATE]

Why I don't advice adding music that automatically start to a blog or any other websites

When you install an invisible music player, that means there will be no button to click to start the music. Therefore you must make the music play automatically for it to make any sense. That is why there is the attribute autostart="true" in the script.

A surfer may visit your site in the middle of the night while others are sleeping, or with a sleeping baby beside him or her, or in a public library, or he or she may already have his or her favorite music playing in the background. Once your music start, the visitor may be forced to quickly surf away from your site to stop the music and you lose a visitor. And if a surfer has already his or her favorite music playing, your music will replace that, and that may cause irritation. There will be no button anywhere for the visitor to click to stop the music.

Still want to install an invisible music player. Well, everyone have their own preferences and opinions and what they think is good or no good. The choice is yours.