No speakers on the monitor. Funny thing is I've been using this monitor for 10 years and, thanks to you I just found out that it has a volume control. So I lowered it to "0" and so far that popup has been gone for the past few minutes.I'm going to guess that your monitor has speakers and that is the volume slider for those speakers. Could be the volume up button is defective and making intermittent contact (as if you were hitting the volume up button).
Cheesy as it sounds, jiggling the volume up button may correct this. Another work around would be to go into your monitor setup and disable sound (assuming you don't use the monitor speakers that I am assume are there).
Could also be that I am way off base.