Convertibles aren't conducive to great sound - at least not when you're speeding along in one with the top down. But with its unique, three-piece retractable hardtop, the 2006 Volvo C70 (sticker price $47,307 as tested) isn't a typical convertible - and the Dynaudio Premium Sound package that's available as a $1,550 option isn't a typical OEM system, either.