Saeviomagy:
Excuse me but no. Any character built around this concept would have taken that into account. If I'm casting 8th level spells then I can hit a target with a medium range spell from 250 ft (Cloudkill, Confusion, Glitterdust, Hold Person, Evard's Black Tentacles, Dispel Magic, Dominate Person, Stinking Cloud, Web, Wall of Ice, Wall of Iron, Wall of Stone, etc.). I can hit them with a long range spell from 1000 ft (Fireball, Pyrotechnics, Silent Image, Telekinesis). Both are more than sufficiently far away to conceal myself in some fashion and make it more than difficult to hear my verbal components. Under the (broken) spot/listen check rules, I dare say that even an epic level character is going to have a hard time noticing me from long range - but any reasonable rules are going to make someone 300+ yards away with 75% cover or better a tough thing to spot. And unless it is perfectly quiet in the environment, even if they did hear my distant verbal components it would be an extraordinary feat to hear and distinguish the sounds of spell casting over other normal sounds of the city or any other noisy environment. I doubt any ordinary character could make the combined listen and spellcraft checks necessary to do it, and that's assuming you didn't force a wisdom check to notice the sound as unusual in the first place.