Suggestions by @jessiebrewster

C.H. Greenblatt

as Kimchi in Chowder

Tara Strong

as Truffles Daal in Chowder

Dwight Schultz

as Mung Daal in Letter L

Dana Snyder

as GrandPat in Letter L

Alex Borstein

as Lois in Letter L

Rodger Bumpass

as Squidward in Letter L

Tom Kenny

as Gary in Letter L

Liliana Mumy

as Panini in Letter L

Mindy Sterling

as Endive in Letter L

Nicky Jones

as Chowder in Letter L

Mike Judge

as Butthead in Letter L

Stephen Root

as Stilton in Letter L

Will Shadley

as Gorgonzola in Letter L

Tom Kenny

as Spongebob in Letter L

Dee Bradley Baker

as Plex in The Squarepants Mystery

Mr. Lawrence

as Evil Toodee in Mystery Mansion

Erin Pearce

as Brobee in Mystery Mansion

Emma Penrose

as Foofa in Mystery Mansion

Adam Deibert

as Muno in Mystery Mansion

Clancy Brown

as Count Krabula in Mystery Mansion