missing sound in hl2

My problem is that at any event where the sound produced would come from an in game monitor/tv screen, no sound is produced. Any idea why this might be? could it be my integrated sound card? im stumped 8O
I have tried reinstalling the game to no avail.

thanks for any help i get.

Model: Dell Diension 1100
Processor: Intel(R) Celeron(R) CPU 2.53GHz
Memory: 1022MB RAM (2 x 512MB Crucial memory)
Hard Drive: 76 GB
Video Card: GeForce nVIDIA 5200 256MB
Monitor: Dell E176FP LCD Flatpanel 17"
Sound Card: SoundMAX Digital Audio (integrated)
Speakers/Headphones: Logitech R-20 2.1 sound system and Dell A215 Speaker set
Keyboard: dell slimline
Mouse: Logitech MX600 Cordless 2.4GHz RF Laser Mouse
Mouse Surface: My Desk ^_^
Operating System: Windows XP Home Edition Service Pack 2