How can I play audio from an SD card?
There are several audio products/shields.  Because you mention "speakers", the first one listed can come with an on-board amplifier to drive speakers directly.  Most shields just give line-out to drive a pre-amp.


Adafruit Music maker shield plays MP3/WAV/OGG from microSD http://www.adafruit.com/products/1788
Adafruit Wave shield plays WAV files on SD card http://www.adafruit.com/products/94
SparkFun MP3 trigger board supports 256 tracks and a serial protocol for running individual tracks. https://www.sparkfun.com/products/11029
SparkFun MP3 shield board plays MP3/WAV/OGG https://www.sparkfun.com/products/10628