If  you want to play mp3 files from your web application then you can do it by using SoundManager2 plugin.
There are few steps that you need to follow.
Step 1: Install the soundmanger2 plugin.

ruby script/plugin install http://soundmanager2.rubyforge.org/svn/soundmanager2

Step 2:  Generate the soundmanager2 in your app. This will store the necessary files in your app.

ruby script/generate sound_manager2

Above command [...]