hey Julius! i guess you want to embed audio using wordpress' standard/built-in mp3 player. That one doesn't work in lay theme currently, but I believe its a bit ugly anyway.
Anyway, to get an unbranded audio player, please upload an mp3 into your media library. Then click on that mp3 to open the information and copy the "URL".
Now in the gridder please go to "+more" -> "+html".
<audio src="YOUR-URL" controls> </audio>
Here is a very technical explanation about the "audio" tag: https://developer.mozilla.org/en/docs/Web/HTML/Element/audio
Here is a list of embeds that should work (in case you dont want to go for the unbranded audio player):
this includes spotify. If your song is played through spotify you get some money for that no?