zvučno

1. resonantly; resoundingly; loudly
   Zvučno se nasmejala i svi su se okrenuli. — She laughed loudly and everyone turned around.
   Vrata su se zvučno zalupila za njim. — The door slammed shut loudly behind him.
2. (before an adjective) acoustically; with regard to sound
   Stan je dobro zvučno izolovan. — The flat is well soundproofed.

A very common collocation is "zvučno izolovan" (soundproofed).