If you've got loads of MP3s knocking around you may want to convert them so you can set them to be your ringtone or text message sound. There's a few things you should remember about doing.
1) Keep ringtones SHORT - less than 30 seconds (phones usually drop to answer phone after 30 seconds)
2) Keep the bit-rate low, so that the file size is (ideally) less than 300K
3) Use excellent FREE MP3 to WAV converting tools such as those listed on the site here or check out downloads section for ABC Audio Converter which does the job nicely.
4) If you've converted your music file, use "Sound Recorder" (this comes part of any Windows installation and can be found in your "Accessories" or "Entertainment" folder). To shorten the file and reduce the bit rate so that it's a smaller file (see point 2)
5) Now copy across to \Storage\Application Data\Sounds and you're done!! :)