White noise from center speaker

At first my original 5.1 sound system altec lansing ada 995 was replaced by dell because my speakers were crackling and popping like crazy. however, the new system 7.1 creative inspire t7900 was also crackling and popping so dell replaced the sound card: sb audigy2. the new card eliminated the crackling and popping but the center speaker started to hiss so yesterday dell sent me a new sound system but the horrible hissing noise hasn't stopped. Why the hiss? What can I suggest to dell? can the audigy2 card handle 7.1?

syrk

Dell-Dimension 8300 Series
Date of Birth September 9, 2003
Chipset PCI Intel I875P at 533 MHz (support processor with bus system at 800 MHz)
BIOS Version: A07
Memory DDRSDRAM Dual Channel
BUS AGP 8x, 8 USB 2.0, 4 PCI
10/100 Pro Ethernet (RJ 45 only)
Intel Pentium 4 at 3.2 GHz, bus system at 800 MHz
2048 Mo DDR SDRAM (4 x 512)
Floppy Drive 3.5"/1.44 Mo
240Go Serial ATA RAID 0 Stripe (2 hard disks x 120 GB 7700rpm with DataBurstTM cache)
Dell Monitor 18" 1800FP Flat Ultrasharp (DVI)
Video Card ATI Radeon 9800 Pro 128 Mo DDR (exit TV support DVI and Dual Display)
Burner DVD + RW 4x/CDRW (12x/10x/32x) and Player DVD ROM 16x with decoder software MPEG 2 (2 players 2bays 5"1:4)
Sound Card Sound Blaster Audigny 2 (Port IEEE 1394 included)
Altec Lansing ADA995 - 5 x 20 W and base 100 W (THX)
Modem Card (DATA/FAX) 56kbps V92