It's a little hard to give advice based on nothing, but saying that it's muffled is a dead giveaway that it's one or both of two things; background noise and/or cheap microphones. It would be helpful to share an example either as like a soundcloud showing your muffled UTAU syllables next to your actual recordings... or just send me the voicebank and I can take a look

Is it certain syllables that are problematic or is it the entire voicebank?
just so i wouldn't have to spend an hour going through every. single. recording. ^^;
Edit: just read the title LMFAO;;;;
Anyways.
When a voicebank sounds muffled it's almost always because your mic was picking up background noise like a computer fan or other noise going on in your house. Then that background noise just gets amplified when rendered in UTAU. That's probably the case, but I don't know for sure since you didn't share an example :/
Ideally you want as little background noise as possible. (That sounds obvious but it's even more stressed because UTAU and most resamplers will pick up on almost all sound!!!)
If background noise isn't the issue then it's probably your mic... but again no example so not sure. Cheap microphones don't handle consonants like s' and t's well. Like using your webcam mic, instant death sentence <3
Is it okay to ask what kind of recording equipment you used?