Mutter, mumble and murmur may look similar, but don't be fooled.
Think of it this way. If someone you're dating tells you they love you for the first time, which would you prefer?
1) "I love you," he murmured.
2) "I love you," she mumbled.
3) "I love you," he muttered.
Okay, none of these scenarios instill a lot of confidence when it comes to long-term relationship potential, but one certainly seems worse than the others.