During a presentation, you might want to have a music playing in the background, then have additional sound effects. The problem is, when the sound effect starts to play, your background music stops. What we want is to have multiple sounds playing at the same time in PowerPoint. Unfortunately, when one starts, the other stops. Here’s why:
When you have multiple sounds embedded, PowerPoint will only allow one sound to be played at a time. On the other hand, if you have the sounds linked, it will be able to play simultaneously. Thus, if you have two sounds, the solution is to have one of them linked instead of embedded.
For instance, you can have either both the background music and sound effect linked, OR, have the background music embedded while the sound effect linked.
Microsoft has addressed the issue here, including detailed steps on how to go about doing it:
“Background sound stops playing in a presentation when the presentation reaches a slide that uses built-in custom animation with sound effects in PowerPoint”