Your Xperia R1 Plus sports a microSDXC memory card reader. If your laptop or personal computer can also read that card format (Most laptops are so-equipped), you should be able to transfer a music file (The Sony Xperia R1 Plus supports these formats: Yes) from your computer to a microSDXC memory card, and then, from the memory expansion card to your cell phone. Your manual should explain how to assign the file to your phone's ringer.
The Sony Xperia R1 Plus is an Android smartphone and the Android market offers a bunch of free ringtone apps such as MP3 Ringtone Maker, RingDroid (open source) and more.
Since your Xperia R1 Plus has a sound recorder (also known as 'voice memo'), a very simple technique involves simply placing your phone nearby your computer's speakers, playing whatever song, music or sound you'd like as a ringtone, and recording it -- you can even use your voice, your child's voice or even your cat's 'meow!' Record it with your mobile and 'voilà!' Unless that function was specifically blocked on the Sony Xperia R1 Plus, you should be able to navigate to the ringtones section and assign that recording as a ringtone. Of course, the sound quality will not be ideal but, at least, it's free!
Here are some of the free ringtones (compatible with your Sony Xperia R1 Plus) we offer:
Free Horror Ringtones Free Ghost Ringtones Free Ringtones-a-gogo Free Techno Ringtones Hear All Our Free Polyphonic Ringtones