I think it may because the original sound file did not copied to other people's machine, while it can be played on your own machine, as there exist source sound on your local C: drive, but other people does not have.
Therefore, if you want to play the PowerPoint file with sound on other people' computer, i advise you to use some PowerPoint Burner like Moyea PPT to DVD Burner
to convert your PowerPoint file to video file in formats of MPEG, AVI, MOV, MP4, etc, and then upload it to some video share web like YouTube to share with the people you like.
Besides, you also can use this PPT Burner to burn your PowerPoint to DVD disc directly, and then distribute the disc to the people you want to view it.
As this PPT Burner can fully retain the original elements, including movie clips, video and audio file; as well as it can add up to 5 background music for the output.