The Flipper features a microSD memory card reader. If your personal computer can also read that type of card (Most notebooks are so-equipped), you should be able to save music files
If the above-mentioned tips fail or if you prefer not to buy new accessories, you can always use one of the numerous free '3G Uploader' services available online. A search on Katapulco should retrieve plenty of them. 3G uploaders work as follows: Transfer your own ringtones on their web site and they will provide a URL. Navigating to that URL with your Plum Flipper's browser will download the free ringtone.
Here are links to some of the free ringtones (Plum Flipper-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones