There are times when it's difficult even for native speakers to understand what words are being sung in a song. When people sing what they hear rather than the correct lyrics, this is called mondegreen.
A good example of this can be seen on the video of the song "All by myself" which has been changed to "Obama's Elf". Listen to the clip; which words do you hear?
If you want to find the correct lyrics of a song just go to www.youtube.com and write in the title of the song and the word lyrics. E.g. All by myself lyrics.
Many thanks to Jamie Keddie for introducing me to this clip.