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1 - If you're going to sing, you should learn to project your voice. Singing quietly rarely happens in a stage or a performance situation. Occasionally in studios, but still, what you're doing in that video is holding your voice in. You're also singing from your head the whole time. Try to sing from your diaphragm and make use of all the air in your lungs.
You don't have to scream, but hunching over, singing in head voice, and singing quietly are all surefire ways to ensure you never really get to grips with your voice. Sometimes you WANT those things, but the majority of singing is done from the chest and with projection, trying to push your voice outwards rather than just trying to hit the right notes.
2 - Decide if you want to sing smoothly or scratchy. There are techniques to mastering both. Be careful with scratchy. You can hurt your vocal chords if you do it wrong for too long.
3 - If you make another video, try and imagine yourself singing to someone at the BACK of a crowd. Your voice needs to reach that guy just as much as it does the guy at the front. (This actually applies even if singing with a microphone, but thats more about mic control than just singing straight up)
4 - Listen to this:
Non piangere Liu - Marco Zanini - YouTube
Yes, he's a guy, and yes, its opera, not rock music, but listen to what his voice is doing. He has power behind it. He's singing as much from the rest of his body as from chest and lungs and throat. Thats the difference between projecting and not projecting. If you're imitating Kurt then none of that **** matters. But if you want to be a good singer (and Kurt was NOT), then this stuff is important.
Remember, there IS a right way to sing. Sometimes even the best performers ignore that, for the specific effect a different way might produce, but you should be able to do both.