Welcome to the forum.
The lack of Audio in an Exported/Shared file is often the lack of the correct setting in the Multiplexing tab. When you go to do the Export, you will choose both Audio & Video in the General tab. Additional settings appear in the Video and the Audio tab. Last, there is the Multiplexing tab, and one must choose one of the other choices, besides None. This is often DV Audio, but there can be others, depending on the other tabs' settings. Check your Multiplexing tab, and let us know what it's set to. That is likely the issue.
This problem can also be related to your source footage.
What type of camcorder did your video come from and how did you get it into your computer?
Can you check if the exported file has .mpeg extension or not. There is a slight chance that multiplexing might have failed leading to no audio in the clip.
If this doesn't help, you can go back to the same export preset, click on Advanced and under the audio tab change audio codec to PCM, save this preset and export again. Now check the exported file, it should be fine