Thank you for bringing this up. Some people honestly don't know... Some people choose to ignore the fact that composers and songwriters do what they do for a living.
I'm not saying this in a harsh way at all, but using music which you have not paid for \ licensed would be like walking out of a restaurant without paying for your meal... or simply walking out of a grocery store without checking out. Legally, ethically, morally, it's theft.
Great music can be done on a budget. Go through the proper channels, get an original work tailored to your film, and everyone wins.
If nothing else convinces you, what goes around comes around. Take care of your musicians, and they'll take care of you.