Please help! Audio Trouble! :(

Well i bought a ati radeon hd 3650 the other day, i got it installed and such, everything is up to date, and when i went to go play call of duty 4, i got no sound what so ever, so i checked the audio and sound devices program and it said ATI HD rear output, so i checked the graphics card and it had a HDMI output on the back of it, do i have to go buy a DVI > HDMI cable? please help, i would like to hear the awesome sounds of crysis and call of duty 4!