Home Theater Enclosure Problem?

Absolutely. It has little absorption. Depending on the size of the room you probably have reverberation times that are triple what you would like to have. In addition to that the pine could have a resonance frequency that creates more problems.

Get some room tunes!

Wood resonates. That's one of the reasons why it's a pretty good building material for acoustic instruments. You're probably getting resonances at critical mid-range frequencies, where most vocal frequencies reside and where the human ear is particularly sensitive.

For starters, put properly rated Vibrapods under your player and pre/pro or receiver. These things are amazing for the money for cleaning up microvibrations and sound-smearing resonances.

What kind of speaker cable are you running to your center channel? And what kind of center channel spkr are you using and how is it sited/positioned?

Wide-bandwidth interconnects help, too. Without spending too much money, try a Kimber Timbre or Hero interconnect for your center channel signal chain (but do the vibration control first).

An enclosure around your system will boost the bass response from the speakers (if you soffit mount). Pine wood would not be my first choice. My first choice would be heavy MDF board for cabinetry with a nice hardwood for the visible surfaces (for aesthetcis and such that it does not vibrate ...dead sound when rapped with your knuckles).

None of this will help with Mid range intelligibilty? Are you using phantom? The center channel information needs to go somewhere if youu don't physically have a center.