My father is 79 yrs old, and all he wants too be able to do is turn the TV on and off, and adjust the volume. None of the other BS that most of us here are looking for.
I have had Samsungs, and they produce pretty good sound from just the TV speakers. My father has a Samsung 55D6000 LED, and sound from the TV speakers, when set to MOVIE or AMPLIFY, sounds pretty good/loud. He enjoys it, and he can set the volume at 15(out of 100), and hear it perfectly fine(sitting about 14 ft away in a LARGE LR).
He has a little trouble hearing, but is not bad. I have found that Samsung and LG "tend to" have the best sounding speakers from the TV. My main experience is with Samsung, and they tend to sound really good from the speakers. A SOUNDBAR or 5.1 surround sound are much better, obviously. But for most people, especially older, they do not care.
I would look at Samsung. I have one myself in my bedroom(UN46D6000). Now, I have a sound bar, and headphones(for late night) for my sound.
But I have the TV's internal sound set to AMPLIFY, and the TV VOLUME set to NORMAL(this is for those occasions when the commercials are BLARING and the sound from the tvshow is lower. This just evens it out, and is best when used IMHO).
I can set my sound to 15-20(100 max), and have NO problems hearing it clearly. No DISTORTION, or vibrating. I would try Samsung F series(2013 models). Depending on what size you are looking for, I would try either the F6300 series, or the F6400 series(more expensive. 3D)
Just go to Best Buy, or SEARS, and try them out. Using the remote, make sure they have AMPLIFY(or MOVIE mode. I like that as well), and the TV SPEAKER SETTING in the MENU under SOUND.
That TV should do your grandparents just fine. But STICK WITH the F6300 series, or above. The lower ended Samsung's(EH, or H), are just not that reliable IMHO.
Here are two from Best Buy. The 55" is about $200 out of your range, but the 50" should be in there. Also check SEARS/COSTCO/WALMART for the cheapest price. You "may" be able to find the UN55F6300 for $999, but it will be hard/tight:
Samsung 50" UN50F6300 :