Had you mentioned that you had the another company Boxed product, my reply to your problem would have been different. I would have recommended that you try this - Go to <FSX root>\Captain_Sim\757 and open the sound.cfg file with notepad, and change the following lines (right at the top) from this:

[Common]
Refresh.Time=10
Sound.Directory=Captain_Sim\757\sound
SoundOffWhenSimIsSlew=1
SoundOffWhenSimIsPaused=1
SoundOffWhenSimDialogIsOpen=1
SoundOffWhenSimSoundIsOff=1

to this:

[Common]
Refresh.Time=10
Sound.Directory=Captain_Sim\757\sound
SoundOffWhenSimIsSlew=0
SoundOffWhenSimIsPaused=0
SoundOffWhenSimDialogIsOpen=0
SoundOffWhenSimSoundIsOff=0

And see if that helps.