Weird sound problem on MacBook

This is a 13 inch MacBook, 2009, 8 GB ram. OS 10.10.3

This perplexing story starts with one application, MuseScore version 2.0.3. It creates a musical score and will play it back. Sometimes, it stops playing sound, otherwise it works normally. If I look at view:synthesizer, the visual indicator acts like sound is playing normally, but no sound comes out. When the sound stops playing, I hear a sharp "pop" over the speakers. I also hear the pop when certain windows open and close, when I quit the application, or restart it. I never know when the sound will stop working. Usually it will stop, with a pop, if I put the machine to sleep with the application open, then wake it up, but not always. Sometimes this happens when I open the application. At other times, for no apparent reason.

It's an open source app. The developers don't seem to know if this is a common problem. It has persisted through several versions, from beta to 2.0.3. It was not an issue with MuseScore 1.3. Quitting the application and re-launching it does not make the sound play again. Usually, but not always, restarting the MacBook will get the sound working again.

So far it sounds like a bug in the application, and maybe it is. Or I might have a hardware problem on my machine. I understand that. But maybe not. So please read a bit further. I'm hoping for a more precise technical understanding of what is going wrong, partly so I can report back to the developers. If I need to fix or replace my machine, I'd like to know about that, too.

Once the sound stops working, if I quit the application, other applications on my machine also refuse to make sound, and they also make a sharp pop at times. Take Safari for example. If I try to play a youtube video, I get no sound. Same goes for iTunes, and so on.

If I go to system preferences;sound::output, I find two output drivers -- Internal speakers and a video projector I own. If I select one, then the other, and so on, I hear the same sharp pop each time I click on the output device. Sometimes I can get the sound working by going to system preferences;sound:sound effects. If I try clicking a few times on system sounds, the sound will suddenly start working. Safari will play again. Sometimes, if I click play, then pause, then play again, in, for example, Safari, with a youtube video, the sound will suddenly start working again.

This does not seem to work while MuseScore is open. Once I get the sound working, I can re-launch musescore, and it plays just fine... Until the next time the problem occcurs.

Someone with strong technical skills might be able to read the foregoing explanation, admittedly complex, and say, "Aha! I know exactly what it going wrong." That would be great. Maybe I can find out what is wrong with my machine, or maybe I can report back to the developers, to help them find and fix a bug.

Thanks in advance.