By default, verbal components need to be delivered in a strong voice. However, the game has several options which may alter this.
It certainly looks like the Cheat spell's "fluff" text was written with that Sleight of Hand check in mind, as it's a very good correspondence in the RAW.
- Sleight of Hand skill use (Races of Stone, page 133):Check: When casting a spell, you may make a Sleight of Hand check to make your verbal and somatic components less obtrusive, muttering magic words under your breath and making magic gestures within your sleeves.
- Conceal Spellcasting skill trick (Complete Scoundrel, page 85)
- Disguise Spell feat (Complete Adventurer, page 108)