Mobile phone problem with playing music

I have a Sony Ericsson w205. I downloaded some mp3 music from the Internet, loaded to the phone. Appears nicely, can be played while browsing ringtones, but can't be played with it's walkman application. It's not showing the title (just some funny characters) and displays a "Failed" message if the play button is pressed. Only one track can be played but I haven't found any difference between that and the others.