FYI, 5e actually has official rules for audible detection distances. The caveat is that they're only present on the Dungeon Master Screen.
Audible Distance- Trying to be quiet 2d6 x 5 feet = 35 ft
- Normal noise level" 2d6 x 10 feet = 70 ft
- Very Loud: 2d6 x 50 feet = 350 ft
These values could be used to inform detection circles' distances.
Typical stealth activities like hiding, sneaking, pickpocketing = you have to make a stealth roll if you come within 35 ft. Farther than that, you're fine.
Spellcasting (with verbal components) rules as intended seems to disallow whispering ("particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion"; the existence of Subtle Spell Metamagic), so you have to make a stealth check if you're between 35-70 feet, and you're automatically noticed if within 35 feet. I wouldn't recommend having two visible circles below each NPC representing 35 and 70 ft; the 70-ft circle could be invisible and a nice surprise if players attempt to cast a (non-offensive & invisible) spell while hidden!
More generally, I'm a fan of a combined sight cone plus a (smaller) audible circle.
