Since you have the memory card, assuming you have a Windows pc - the easiest way I have found is with the memory card installed in the phone, turn the phone OFF, and then hook up the USB cord. 

Your pc should see the memory card as a removeable storage drive - so just copy/paste your music to that disk drive.  Then be sure to right click on and do the "disconnect removeable storage drive safely" down in the task bar when you are finished.

With the phone turned off and connected to a windows PC  the memory card will be detected as a "removeable disk", just like an external drive or a USB memory stick. 

As intended.

if ur w580i came with an installation cd, or not, install disc2phone from the sony ericsson site. just install it and then open up your music folder, click on the music u want to transfer, then drag it to the disc2phone window and there, put as many songs and then click on transfer. your music should be transfered to your w580i. when everythings done u should get a confirmation message saying ur songs were transferred sucessfully

--OR--

 as other users say

just plug in the phone to ur computer. after u plug it in go to my computer. the memory stick should show up as drive G, lcick on it, then there should be a bunch of folders. click on music. then drag-and-paste ur music.

but doing the method with the disc2phone program is much better because if u do it the way other users tell u some of your music may not appear on the walkman player, and will only show up as ringtones

I haven't had any issues with my music not showing up in the Walkman or only appearing as a ringtone with using the method I suggested - so long as you place it in the music folder on the memory card (but not the ringtones folder), it shouldn't be a problem. 

I'm not advocating against Disc2Phone - I just don't know why anyone would want to use more steps than necessary when it's a simple copy/paste from Windows Explorer.

My solution to my new Sony Ericsson 580i with AT&T's propietary software that doesn't work ?........ Its now in a bag in 10 pieces after I threw it against the wall. Unbranded, the phones work fine, but with all the bull that AT&T puts in it, it worthless. I bought a new one UNBRANDED and UNLOCKED and its great.

Never buy a phone with manuf features from AT&T.

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fatwilly wrote:

My solution to my new Sony Ericsson 580i with AT&T's propietary software that doesn't work ?........ Its now in a bag in 10 pieces after I threw it against the wall. Unbranded, the phones work fine...

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Your solution of destroying the phone is ludicrous.  Not many of us here will ever claim the W580i is a particularly good phone, but with a slight amount of effort and zero money one can debrand and enjoy.  There is even a tutorial thread in this forum!   Hope you like your pile of electronic scrap.

{Please keep it courteous and edit out any uncalled for comments.  Thank you.}