Windows 7: Need help with Sony vegas pro 9.0

Hello there,
I haven't used my sony vegas for months now.
I've recently decided to edit a couple of videos and I came across this problem:
When I'm inserting audio, say an mp3 file (file->open), and drag it into my video, the audio track would show up as normal(timeline, working mixer):




But there is actually no sound for some reason whenever I click "play" in sony vegas, it's like if it was muted

If I reneder it though as an .AVI file, for instance, the .avi will be working fine (playing it in windows media)

Please help, would be really grateful!
Thanks in advance,