They should work fine and at any time when you press the buttons regardless of where the 757 is. Try editing the sound.ini file. The file can be found in the FSX\Captain_Sim\757 folder.

Change 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

See if that helps with your sound. Backup the sound.ini file BEFORE making any changes!

In v4.6, you should not need to do this, but you do need to do it if you are using v4.4.