First off, you do not know what you are doing. Sorry if that seems harsh but you are going to hurt yourself. You friends who seem to be able to sing w/o lessons maybe are not as good as you think.

I can address some of the causes of your described problems but my advice is...

Get lessons from a trained vocalist ASAP.

The muffled sound could be due to a few things. I am guessing because your description is not enough to diagnose. It could be that your mouth is closed too much preventing the sound from getting out. Trained vocalists will sing with a kind of "smiling" or "snarling" face, with the cheeks pulled up. That's not to look happy or evil (in the case of metal singers) that is proper mouth posture. Second, it the sound "feels" stuck in the throat then you are not properly supporting the sound with your diaphragm. When you learn to use that you realize that the THROAT DOES NOTHING. Your vocal folds are like reeds, you don't tighten your throat you blow air over it and sound comes out. Trying too hard to FORCE your throat to do anything will create tension that will (1) generate crappy sound, (2) potentially cause damage to your vocal folds (chords), (3) Cause pain in the larynx and back of the neck. None of that is good.

I would not suggest any warm ups for you at this point as I think they could be misunderstood. It sounds like you need some basic vocal lessons and to learn how to support sound with your breath, how to relax the throat, and how to avoid "covering" the sound with the mouth.

As for range. It takes time to find your full range. As your technique gets better you find that you can go higher and lower. When I started I had a 2 octave range and now it's 3 and a half, going down to D3 I think and a 4th above high C.

First off, you do not know what you are doing. Sorry if that seems harsh but you are going to hurt yourself. You friends who seem to be able to sing w/o lessons maybe are not as good as you think.

I can address some of the causes of your described problems but my advice is...

Get lessons from a trained vocalist ASAP.

The muffled sound could be due to a few things. I am guessing because your description is not enough to diagnose. It could be that your mouth is closed too much preventing the sound from getting out. Trained vocalists will sing with a kind of "smiling" or "snarling" face, with the cheeks pulled up. That's not to look happy or evil (in the case of metal singers) that is proper mouth posture. Second, if the sound "feels" stuck in the throat then you are not properly supporting the sound with your diaphragm. When you learn to use that you realize that the THROAT DOES NOTHING. Your vocal folds are like reeds, you don't tighten your throat you blow air over it and sound comes out. Trying too hard to FORCE your throat to do anything will create tension that will (1) generate crappy sound, (2) potentially cause damage to your vocal folds (chords), (3) Cause pain in the larynx and back of the neck. None of that is good.

I would not suggest any warm ups for you at this point as I think they could be misunderstood. It sounds like you need some basic vocal lessons and to learn how to support sound with your breath, how to relax the throat, and how to avoid "covering" the sound with the mouth.

As for range. It takes time to find your full range. As your technique gets better you find that you can go higher and lower. When I started I had a 2 octave range and now it's 2 and a half, going down to D2 I think and a 4th above high C.