I found out that with sony ericsson phones you cannot play mp3 ringtones, but what you can do is, you can find a mp3 to wma converter program, such as "power converter" the use it as ringtone, alert, alarm... whatever you want.
Power converter is free to download and allows you to convert 1 at a time, but if you wanna convert many at once you need to buy it, so just download it and convert 1 at a time... I think thats the loophole!!
i hope i helped.