How do play sound in this project?

So for my first project, I want to create something that plays various theme songs every time a door is opened. For the sensor, I have a reed switch that is fully functional. For the songs, I have an MP3 Module which takes in MicroSD and loads the file.

http://www.dx.com/p/micro-sd-card-storage-memory-board-module-for-arduino-146706?Utm_rid=52746118&Utm_source=affiliate#.VnDbIvmrShc

However, how do I add a speaker to this set up? Is there a limit on how loud the speaker can be? Is there a way to use any speaker that I want?

I've been told about a "amplifier"? What is that in terms of product?

I've been told about a "amplifier"? What is that in terms of product?

[u]Here[/u] is an example of a small-inexpensive audio amplifier.

Or you can use "computer speakers", which have an amplifer built-in. (The "clue" is, computer speakers plug-into the wall, or they have a power supply that plugs-into the wall.)

Or you can get a power amplifier chip (such as the LM386) and build your own amp.

http://www.instructables.com/id/Blinking-LEDs-to-the-Frequency-of-Musi/?ALLSTEPS

http://maxoffsky.com/maxoffsky-blog/how-to-play-wav-audio-files-with-arduino-uno-and-microsd-card/

How do these two do it?

How do I connect a computer speaker to the circuit?