Someone get me a pencil and a piece of paper!
The comedy ideas are coming to me fast and furious. (Statement not associated with the mega money making Vin Diesel franchise.)
Find an old silent movie or an old silent short, (Yeah, I already know that if it's silent, it has to be old!), delete the title cards, remove the music, dub in the actors voices so that they now can SPEAK, come up with a comedic story that follows the visuals, add some Foley sound effects, add some new music if needed.
The result is guaranteed to bring forth rip roaring riotous laughter from all Rifftrax fans.

I heard that when the Rifftrax writers read my previous suggestion, they threatened to go on strike because it would mean more work for them. Let those lazy hacks go on strike, you can just go to the front of Home Depot anytime, and you can find plenty of people ready willing and able to write for Rifftrax. They probably are much funnier than the current Rifftrax writers, and best of all, they love working overtime and they work...CHEAP!
If that threat works, you could raise the Rifftrax writer's pay all the way up to minimum wage, give them microwave privileges in the break room, add 5 minutes to their monitored bathroom breaks. That should be more than enough to bring them back to their senses.

Vote YES on this idea to bring sound to the silents!