Leave the cap in place, it's used for direct porting out of the signal. If you remove it, your speaker connections go silent.
Use STBY/LIST on your Beo4 to access the additional menus, go to ADD, and add AUDIO SETUP if you don't have it already in LIST. Afterwards you can access AUDIO SETUP using the LIST button and make the adjustments you want to startup volume, bass, treble and balance.