Parents need clear sounding 2 ch HT

Actually had some experience with this a couple of years ago while working at an A/V dealer.
Jmcgrogan2 is on the right track, however it seem to work best IME, to boost the treble more than the amount of roll off in the bass. I think this is due to the fact that, the more a treble control is increased, the band of affected frequencies generally widens, resulting in a boost much lower than the center point of the control. Also, we gradually lose our high frequency hearing as we get older.

This should help, however, if not enough, follow Marty’s advice with a center speaker.

If that’s still not enough, have their hearing tested.

Elizabeth,
Would this one work?

http://app.audiogon.com/listings/canton-ergo-cm-402-center-speaker

ditto Marty and Kal

More likely high frequency hearing loss if that's the issue. While voice occurs in midrange, the small cues that isolate and distinguish sound and forms subtle articulation cues occur at pretty high frequencies. Talk with any retired dentist about why they can't process conversation but have normal hearing through the midrange.

Quite simple solution that will cost you around $50.00. TV EARS work and will stop the "what did they say" and your folks will be happy!