Hi Andrew, same feedback from here. I didn't find your tone to be too muffled. Its quite nice. Also try what Steven says, standing up will change the sound for sure. Also, being an asian myself, I think there is ever so slight a difference on how we sing English (because its not our first language). So that might also be a reason why your friend says you sound muffled?
you can check out my reverb page if you have some time: http://www.reverbnation.com/ashiquemfahim
Keep on singing!
Fahim