I want to add some audio to the end of a video
It sounds like you are looking to do what is traditionally handled by a Non-Linear Video Editing software, or just video editor for short.  There are a number of free and cheap options available.  They lack many of the bells and whistles of more advanced software, but are actually typically easier to use than the more advanced software targeted at professionals.

If you are on Windows, Windows Movie Maker will easily allow you to do what you are trying to do and has a nice, friendly user interface.  Due to stability issues and limited functionality, if you want to get more advanced than very basic stuff, it is worth purchasing a better option, such as Premiere Elements, Pinnacle Studio or something similar in the consumer targeted market though.  

If you aren't on Windows, you could use OpenShot video editor which is available for Mac and Linux and should also be able to cover your needs.

As Julian was kind enough to point out in his answer, there is also a list of open source video editors listed on Wikipedia's List of Video Editors, though there is a very wide range in the usability of the products in that list, though most all of them should be able to address your fairly basic needs.

From your comment, I noticed that you were doing some powerpoint slide stuff.  You can actually take an image of that and use it directly in the video editor as well, rather than having to output the video through PowerPoint.