Win 7 has some screwy stuff when it comes to it's sound... I have had to help 5 or 6 people with this same probelm and moreoften than not it's the mixer is muted for bf2 or or just turned all the way down. Try clicking in the speaker icon at in the bottom rt part of the tool bar, click mixer...
What version of windows are you using? If it's win 7 then check your speaker settings in the bottom right corner of your desktop. If you click on the speaker then click on mixer, it will give you volume controls for every program that you have open at that time and it keeps the levels you set...