FS9 744 2D Cockpit Sound Problem

I"ve recently bought a new comptuer and re-installed everything including the PMDG744 for FS9. The newest update and sound updtae has already been installed. However, I discovered that when I"m airbone, the 2D Cockpit is having a sound problem. The sound of wind/engines heard is very abnormal. I dont' know how to describe it, but it's like the sound 'chocking' or 'lag'. In VC or Spot plane view there's no such a problem. I'm using the same 5:1 speaker as before and a brand new motherboard with audio function, so I'm quite sure it's not a hardware problem. Is it the newest service pack that caused the problem? Or did I did something wrong in the installation? Thank you.