The next time you sing a song, or give a presentation, become conscious of 'opening up your words'. So many times I'll hear a singer or speaker, and can't understand most of the words they're saying or singing! What you've got to realize is that people may not have heard your song before, so all your words, your entire story, is new to them. If you're presenting, everything you'll be saying to your audience is brand new!
Do your audience a favor and let them CLEARLY UNDERSTAND YOU. No one will be focusing on your message if they're straining to hear your words.
The way to make yourself clearly understood is to open up your words. I don't mean to open your mouth in phony, exaggerated movement. All you have to do is simply make more space INSIDE your mouth. By imagining an orange (or some other small fruit!) inside your mouth, you'll lift up the soft palette and open up the words. Elongating your vowels will also make it easier to hear every word.
It's also important to slow down your speech, or slightly overemphasize your pronunciation. What is clear to you, may still be hard to understand for someone who's unfamiliar with your words and topic.
Singers, especially, need to open up vowels and make space for their words so that lyrics are clear and understandable. If you're telling an important story with your song, you want it to be understood, otherwise, your audience won't be able to relate to your music and you will have lost a very important emotional connection.
Remember that what sounds clear to you in normal speech or singing, may not be clear to an audience who has never heard your presentation or music. Open up your words to clearly communicate your message and the emotion that is the key to your success as an effective performer!