You have to add the new music file(s) yourself by accessing the phones player; once you have that open view the 'walkman' options, listed beneath will be artist, tracks, etc. There you can add an mp3, or all mp3's into a playlist. Each time you transfer new mp3's or other audio/video files you will have to renew(refresh) your playlist in order to view, listen, and manage.
Oh and make sure you are putting files in the correct folder when transferring. There are multiple file folders, which I'm sure you're aware of between the two phone cards, the wrong folder could result in malfunction of processing. There is no doubt about it, that Sony Ericssons W300i is very glitchy, and poorly constructed.