This post-production sound company, Twenty-First Century Entertainment (formerly known as 21st Century Sound Design Corp.) was founded by Joel Valentine (who developed The Producer's Sound Effects Library and was mentor to Spongebob sound designer Jeff Hutchins). They worked on mostly cartoons, mainly ones from Hanna-Barbera and Cartoon Network, usually crediting sound editing only. The company was said to be folded into Hacienda Post/Sabre Media Studios after the end of Sym-Bionic Titan in 2011, although Joel and his uncredited team still do sound design to this very day. One of the (usually uncredited) staff members included Jeff Hutchins, an FX editor on 2 Stupid Dogs Season 1. Jeff assisted Joel and they worked closely together. While "The Producer's Sound Effects Library" is unknown in availability (though probably used by Hacienda Post, Advantage Audio and other popular sound studios for cartoons), Jeff was given both this and Joel's ADAP library from Joel himself (source: this tweet ).
- Jonny's Golden Quest (1992) (uncredited; Joel is credited as the sound editor)
- 2 Stupid Dogs/Super Secret Secret Squirrel (2 Stupid Dogs segments only)
- The Halloween Tree (1993) (uncredited; Joel is credited as the sound effects supervisor)
- The Town Santa Forgot (1993)
- What a Cartoon (many episodes) - Audio Services, Sound Editing
- Jonny Quest Versus the Cyber Insects (1995) - "Sound Effects Editorial"
- Dexter's Laboratory (seasons 1-4 out of 6 only, ending with Dexter's Laboratory: Chicken Scratch (2002), which's featured in season 5)
- Cow and Chicken ("No Smoking" pilot only, from What a Cartoon!)
- Johnny Bravo (first 3 episodes only, rooted from What A Cartoon!) - Audio Services
- The Powerpuff Girls (Seasons 1-6 only, including The Powerpuff Girls Movie (2002) and The Powerpuff Girls Rule!! (2009))
- Dexter's Laboratory: Ego Trip (1999)
- Samurai Jack (All seasons, but uncredited on Season 5)
- Samurai Jack: The Premiere Movie (2001)
- Foe Paws (uncredited; Joel's created for Sound Editing)
- Evil Con Carne/Grim & Evil ("Evil Con Carne" pilot only)
- Codename: Kids Next Door (2001 pilot "No P in the O.O.L." only)
- My Freaky Family: Welcome to My World (2001)
- The Powerpuff Girls Movie (2002) - "Sound Editorial and Design"
- Dexter's Laboratory: Chicken Scratch (2002) (Shorts) - "Sound Effects Editorial"
- Foster's Home for Imaginary Friends (including movies "House of Bloos" and "Destination Imagination")
- Welcome to Wackamo (2006) (uncredited; Joel was credited as "sound editor")
- Korgoth of Barbaria (2006)
- Chowder (TV Series) (2007 pilot only, uncredited; Joel was credited "sound effects editor")
- The Powerpuff Girls Rule!! (2009)
- Sym-Bionic Titan
- Wander Over Yonder (season 2 only; uncredited)
- Big City Greens (uncredited)
- Primal (2019 TV Series) (uncredited)