There are many ways to add music on Tumblr.  From the very own Audio Post in Tumblr itself to the many sites that offer music widgets.

One of the best websites to get an MP3 music player to add on Tumblr is Hypster.com.  Register is easy and the most interesting thing is that they seem to have all the albums from the most popular bands.  You can create your own playlist and choose wheter to auto play or not.  Many tumblrs want to make the original Hypster music widget smaller, so it doesn’t take the whole space in the theme.

How to add and customize your Hypster.com music widget on Tumblr.

• This is the original Hypster.com mp3 player as it appear with the original embed code:

• If you want to make it look smaller, lets say “hide” the whole playlist displaying only the controls and the song name, you need to edit the code. We are interested in the width and height values, there are two pair of them in the widget, adjust them to something like width:160px and height:15px. You should also get rid of the image tag that is at the beginning of the code. Finally, change newplayer.swf for player.swf This is how it will look like after the tweak.