Why not?

[Jessie]

Why can't I just plug my computer speakers into my mp3 player, instead of headphones?  The jack appears to be the same.  Why doesn't it work?

[BigDun]

It does work. I actually have a splitter on my speaker cables so they plug both into the back of my computer and into my IPod. Works great.

[Jessie]

It didn't work for me.

[BigDun]

Try turning on the speakers and hitting play on the mp3 player.

[Jessie]

Oh!  Good idea!

[Jessie]

Weird.  I decided to give it another shot, and it turns out that the key is turningthe mp3 player up all the way.  huh.

[BigDun]

Oh yeah, I forgot to tell you. You have to turn the MP3 player all the way up. Hope that helps.

[Jessie]

You're my hero!

[Bennyhana]

Same plug doesn't mean the same signal.  In this case, the signal put out by your MP3 player was much smaller than the signal put out by your sound card.