If you mean that your ram preview is only 5 seconds then that may be normal depending on what you are doing. If you mean that you can only render 5 seconds of video then your settings may be wrong. If you mean you render the entire video and you only hear 5 seconds of video then you may have an issue with the format you are choosing to render. If, If, If... There are too many iff's to give you a definitive answer because we don't know anything about your project, your workflow, your settings, your system or the version of AE. The only think we can suspect for sure is that you are fairly new to After Effects and that you have either set something up incorrectly or that you don't understand how AE works.
All i did was was drag and drop a song in the project then added it to the the bottom, nothing big. I only have two things in my project a picture and a song.
Because you only have 8GB or RAM to is almost certainly because you don't have enough RAM to build a long preview (as Rick suggested above). What is the resolution of your comp? Try switching the preview to 1/4 resolution.
Render a complete output file to see if it fixes the problem. If your rendered file is too short, your comp is too short. If your rendered file has no audio at all, you've set up the render wrong.
How do you get your MP3 to fully play in AE CS6? You don't use MP3's, that's how!
You use a different application to convert them to uncompressed audio like aiff or wav. AE doesn't like compressed audio.