There's a sound setting in the game... did you look at it when using ISBoxer?

When using the Quick Setup Wizard for WoW, ISBoxer sets up a Character-specific Config.WTF file -- which stores your sound settings, graphics settings, stored account name, realm and character selections. The first time you log into a character using ISBoxer, his Config.WTF file is created based on the current contents of the actual Config.WTF file. So if you had sound off in WoW at the time in the main (non-ISBoxer) config, that guy will have that setting too, until you change it on this specific guy.