Friends send powerpoint presentations to me which have music in the background. Is there a way to easily prevent the music from playing when the file is opened in impress?
Is the sound part of a slide transition? Then you can use the Slide Transition pane of the sidebar. It has a section “Modify Transition” and there a drop-down list “Sound”. Set it to “No sound” or “Stop previous sound”.
Unfortunately there is nothing related to sound or music in the transition section.
The easy way is to disable all sound. All Systems I know have a mute switch, usually located in the task-bar. Laptops have special keys.
So sound is actually played, but you will not hear it…
If you need other sound but want to mute everything from presentations, you could use an audio router app. I use this one, which allows muting/rerouting on app level. It does not enable routing of apps until they start to submit audio, so setting up is not entirely smooth. It does the job…
Also, with software under development, there is always a risk of bugs, and even in some cases malware. Check file with your tool of choice, and use at own risk 
I think I remember that sometimes, PowerPoint sounds are placed as an object on the first slide of the presentation, so you’ve gotta search for that object and simply delete it.