I saved m Audacity file as an Mp3, but when I played it back, the background music was gone and only 1/2 of the actual song played? Is it the program or my computer? How do I fix it? 
Ensure you are using the current version of Audacity from http://audacityteam.org/download/ .
Unmute all the tracks you want to include in the export.
To be sure you export the whole length of the audio, use File > Export… not File > Export Selection… .
Gale
Do you have the latest version of LAME from [u]this page[/u]?
I assume you still have the original file? Did it play OK from Audacity when you first opened it? Did you do any editing or add any effects, or did you just convert it to MP3?
the background music was gone
I’m not sure what you mean by that… What LAME settings did you use?
Thanks Gale! It finally worked!!! 