Since your iPhone 7 Plus has a sound recorder (also known as 'voice memo'), a very simple technique involves simply placing your phone by 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 iPhone 7 Plus, you should be able to navigate to the ringtones sub-menu and assign that recording as a ringtone. Of course, the sound quality won't be ideal but, at least, it's free!
Here is a sample of the free ringtones (compatible with your iPhone 7 Plus) we offer:
Free Horror Ringtones Free Ghost Ringtones Free Short Ringtones Free Far West Ringtones Hear All Our Free Polyphonic Ringtones