How much experience do you have using this application? How do you measure it: in years, months, weeks or days?
It makes a difference.
I've been using After Effects for at least a year, I have not done anything that I know of that would effect how the audio plays or anything of the sorts, it's just not playing anymore.
How are you doing a preview? If it's with the spacebar, you're doing it wrong.
You initiate an audio preview by pressing . on the number pad and you initiate a RAM preview (which includes audio) by pressing 0 on the number pad.
It sounds like you're new to AE. You need to take a step back and start here.
What format and codec is this video?
Did this same video file work before?
What exact version of 5.5 (including all the decimals)?
If you have access to another computer, does it work on another machine?
Have you tried the trial of After Effects CC? You might see if it works in there.