is there a way to use arduino with some old speakers that dont have a board anymore to create computer speakers?
You can sound tones on a speaker using an arduino + resistor(s), but it will be 8-bit music. http://code.google.com/p/rogue-code/wiki/ToneLibraryDocumentation
To play music with a better quality, you will need an MP3/WAV-shield or something similar.
I believe you want to amplify the sound.
Since then, you will not need an arduino but Amplifiers