Salaam,
It is more becoming to keep a lower tone in the presence of people of authority.
It follows that we should extend this etiquette to graveyards out of respect for the dead. Shouting and talking loudly seems disrespectful when you are next to a grave. This should be for all the dead of course.
Adopting this attitude elsewhere would mean that you would try to conduct yourself in a similar manner on a day to day basis. That is,by keeping a medium tone in speech. Even prophet Luqman advised his son that he should lower his voice when speaking, saying that :
"Verily the harshest of voices is that of an ass" (31:19)
Obviously this was his personal opinion but he was advising his son of good things and we should always try to do what is better