you have to jump through some hurdles but here is how i download my favorite music as ringtones. I go to YouTube and find the music I want. I copy the URL then go to the browser and look up "YouTube to mp3". paste the URL and then the video is converted to a mp3 for download. once downloaded you should be able to add the mps to your ringtone folder or add it directly by selecting the + symbol and finding the file.
cropping songs will require a 3rd party app since Android only let's you add the full song or a auto generated snippet.
I use this app for my snips and i have had no complaints: