Do they want/need HD and HDMI? If so, you need to buy them an AVR. I know of no 2 channel receivers that support HDMI. They can keep the same two speakers although adding a center channel might be of assistance in voice clarity.
I think your parents are suffering hearing issues that seem to them as garbled sounds but is really a medical matter. They probably need the speakers near to them and the volume on the louder side for them to be satisfied. I would advise them to get their hearing tested. Then other options aside from what I suggested may be appropriate.
Clearer sound is possible.
Various speakers have clarity as a feature.
I find one of the best are Canton speakers with the 4" aluminum cones. The Canton CD series.
Then a small power conditioner can help clean up the sound.
For an inexpensive means some computer speakers can do.
The speakers in the TV are really nearly always poor quality.
Room treatments will reduce reverberation and improve clarity. When I bought acoustic panels warehouse clerk showed me conference room that they treated 100% with acoustic fiberglass foam (floor to ceiling between studs). Sound was amazing. I've never heard another person speaking with such clarity, sound coming directly from his mouth. It was amazing. Perhaps it is not good for music, being over-damped, but is perfect for clarity.
Further to Kal's comment, I' d add another step (if they're using an AVR).
Add a center channel, use the auto set-up on your AVR and then manually increase the volume level of the center channel. This problem is often a result of effects being mixed very loud relative to dialogue in an effort to maximize the sonic impact of the audio.
Bumping the level of the center channel may reduce some of that impact, but IME It will usually help make the dialog much easier to hear.
While referring a friend to the Zvox family HT speakers, I noticed that they had added a "Dialog Emphasis" feature to their products. I'm sure that this is just a boost in the vocal range but, if you need it, it can be important.
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 Martys advice with a center speaker.
If thats still not enough, have their hearing tested.
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.
My wife and I are beginning to suffer the same fate, with difficulty making out the dialog on TV programs and Blu-ray movies. So, I have been battling this problem for years.
Actually, I find dialog is much more intelligible through the TV speakers than through outboard speakers. I think this is because TV speakers are voiced specifically to that end. But, if you are going to use a receiver and speakers, the speakers need to have really good tweeters and no mid range muddiness or resonances in the vocal range.
I put in a system for my parents with an Onkyo 608 receiver and a set of front L/R Legacy speakers(actually, I put in those Legacy speakers maybe 12 years ago but just put in the Onkyo 2 years ago). It is now a HDMI setup. It just sounds great, and dialog is very clear. Now, it may have something to do with the acoustic characteristics of their "den", as the call it.
For your parents, I would get a Onkyo 609 and a set of Monitor Audio bookshelf speakers or small towers. No need to buy new to get good sound clarity, as long as the drivers are in good shape. Monitor Audio are known for clear sound reproduction. I put a set of Monitor Audio Silver 9i in my two-channel bedroom system, and with that gold tweeter dialog is pretty clear, the best I have achieved in that room.
I'd also recommend, if you get some small towers, that you try to move the towers out into the room and as close as they will let you to their listening position, to optimize the balance between direct and reflected sound.
As a Physician, i agree that this is probably a medical issue. Invest $30 in something called Miracle Ear. It is a headphone attached to a pocket sized gain amplifier. My mother loves it and has gone from being a spectator af family dinners to being a full participant.. She also says it helps her immeasurably understand the TV. We are buying one for my father in law for fathers day.
All the other hi tech stuff misses the point, although a center channel may help
if the receiver allows for a 3.0 setup. I don't see where HDMI would make any difference
Another idea is headphones. I got some Sennheiser RS-220 wireless phones for late night TV/movie watching, since I live in a small house and have little kids who (are supposed to) go to bed early. I find it much easier to hear the dialogue clearly through phones. you can even run multiple pairs, and everyone has their own volume control on the headphones themselves.