[Jack Nguyen]

This is a VERY common thing in AE CS3 and 4…
Simple: Just as Dave said above, here are the steps:

1. Edit > Templates > Output Module
2. Inside the “Settings” area, click “Edit” (The second button)
3. Make sure the “AUDIO OUTPUT” button at the bottom is CHECKED 😀
4. Choose the desire options for your project.

Note: Delete the previous item(s) you have in your Render Queue and add a new one AFTER you change the settings. The previous one keeps the former settings which is “Output Audio” is OFF.

I wish they made it simpler… 😀
Cheers!

Jack at jackfolio dot com