Add Audio to introductory video in Adobe after effects

Import audio file and video file. 

Drag video file to composition window. And drag audio to same composition window. 

You'll see as in above window.

Maximise the Preview window and click the last button above RAM Preview

Saving the final video output:

1)GO to composition

->Add to Render queve

->You can see output module which will be having lossless as default option.Click on it. You will see below window.-->Go to make template

-->Edit

Change Format to FLV for smaller size.

Finaly click on Render.

Enjoy.