Making NPCs come to life is an important aspect of Game Mastering. No matter how well-written an adventure is, if the non-player characters are as interesting as flat cardboard cutouts, your players probably won't become as invested in interacting with them as they should. Your NPCs will virtually become little more than walking experience point totals, and loot drops.
While researching different accents and character voices on-line, we came across a five part series by, DawnforgedCast on YouTube, called Voices and Accents. It starts off with basic human-type character voices and accents, progresses through common monsters, and eventually presents the Diabolical inhabitants of the Nether Realms.
Part 1: Monstrous Voices: The Core Races - Runtime: 20m 30s
Part 2: Monstrous Voices: Female Voices - Runtime: 2m 55s
Part 3: Monstrous Voices: Orcs, Goblins, Kobolds, Giants.. - Runtime: 9m 40s
Part 4: Monstrous Voices: Elementals - Runtime: 5m48s
Part 5: Monstrous Voices: Demons and Devils - Runtime: 8m 22s
After you're finished with this series, you may find yourself interested in their other videos. Be sure to leave them a comment and maybe thank them for posting useful content.