Really, this isn't a "problem."
Some people are quieter than others during sex. Sometimes that's due to sex being new, or having a new partner; sometimes it's just personality.
Bear in mind, too, that it's not uncommon for young adults -- especially young men -- to have elevated expectations of how women will react during sex. The media generally presents women as being very vocal during sex -- even something which isn't super-high-stimulating like vaginal intercourse alone. Some women are that way with some partners and some sex, others are not. Doesn't mean anything is wrong.
Obviously, if sex with a given partner isn't all that thrilling -- or is uncomfortable, which it really shouldn't be if everyone is communicating and responding to communicated wants and needs, even at first -- then explore more, talk about what you like, what feels best.
But if you're already doing all that and everything feels divine and you're just a quieter type in bed? No worries. If your partner is worried, then just talk about this, make clear you ARE enjoying yourself, but are just a quiet type.
Editor & Founder, Scarleteen
ST blog • about Heather & Scarleteen
"You have to love women who are brave enough to do things so big in a world where women are supposed to be so small." - Andrea Dworkin