Everything is ok but no sound out!!!!

Hello,
i think i have so strange problem with my pc (no sound and every thing looks ok and when i talk to my friends by microphone they can hear me but i can hear any sound from them or from any other way...
i have
GIGABYTE
S-series motherboards
Supports AMD sockets AM2+/AM2
GA-MA78G-DS3H

i faced a problem before with my windows XPsp2 and i wanted to change it and before i do i made an image with using Ghost and i did and i installed Vista after that but after that and before to enter any partition or enter the internet i found no sound even i have the original drivers for the motherboard which have the sound and viga (are intell,i mean not external,i mean the motherboard include both of sound and viga) and when i finished i found no sound although every thing is ok and when i checked the drivers it's looked ok and i have got an icon for realtek HD audio Manager and looked is ok and also when i press volume beside the clock look like is ok ,and when i play any sound or vidio looks ok ( i mean i get no any kind of erorr) but no sound out and also the microphone is working !!!

anyway when i was n't able to fix that problem

i used Ghost again and back to my old system with little erorrs

and now i still face problems so i wanted to change it again after 3 weeks but use also Windows SP2
and when i finished i found the same problem no sound out

it's exactly as some one made mute for sound but i check it and it's ok

i don't know where it the problem

i didn't change or move any hardware or what i did put a cd for the Win XP sp2 and put another to install the drivers

and before i installed Win i made quick format for C: NTFS

i have HDD 320 Gb
and AMD Athlon (tm) 64*2 Dual
Core Processor 5200+
2.71 Gh 2 GB of RAM (512 MB for Viga)

i hope if any can give me help

i don't want to back to windows with erorrs
and i don't want to stay without sound

ummmm

and thanks alot

adam