It's not as easy as that. The SD card and playing audio are two different things.
However, probably the easiest way is to buy an MP3 shield (but that spoils the fun to make it yourself).
On the other hand, to make it yourself, takes quite some effort.
Also, the audio quality will be very low.
Besides that, decoding MP3 is far from trivial.
In case you want to make it yourself:
- First write a program to read (stream) bytes from the SD card
- Write an MP3 decoder (maybe/probably there are open source variants)
- Forward the MP3 bytes to the decoder
- Forward the audio bytes to the buzzer
Of course, there are time constraints otherwise you hear gaps in the music.