Note: This is only how I portray their voices, in short: my opinion. If you don't agree, it's okay.
Flowey: Like a young boy with a slight robotic echo
Toriel: Like a 30-year old woman with a slight British accent
Napstablook: Sounds a little like Harry Styles (without the accent) with an "echo"
Sans: Somewhat like the Frostbutt
Papyrus: The same way Markiplier voices him and Itward from Fran Bow
Muffet: Like Chica from Five Nights at Freddy's: THE MUSICAL (see media)
Mettaton: Same way Demonya from YouTube voices her but without the echo and a little lower
Alphys: A little squeaky, somewhat like Sandy Cheeks but without the Texan accent
Undyne: Deep, has a good blend of gentle and threatening tone, and with a slight (almost unrecognizable) Russian accent
Asgore: Same way Wosh Ur Bones from YouTube voices him
Asriel: Same as Flowey but no echo
Grillby: Same as Sans but with a British accent
Chara: A little like Muffet but with a heavy robotic echo
