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.