G said:

I have noticed that when playing TV or DVDs through my stereo that the
people talking can be hard to hear, but music and other sounds are
quite loud. I have tried changing the settings on my stereo receiver,
but nothing really helps. My receiver is a Sony STR-D990 (12 years
old although it has Pro Logic, but not the newer modes). I have 5.1
speakers coming out of it. Would getting a new receiver help the
sound?

I'd bet your center speaker isn't performing properly. Could be the
connection or the speaker, or even the amp feeding the speaker. Try
a channel test if your receiver has the capability.