Easiest way (for me at least) to reproduce is either:
1. Go into Editor - spawn as rifleman (so you have grenades).
2. Throw them at an equal distance and far enough so as not to kill yourself, almost always 1 will be much louder to the point of being really uncomfortable to hear.
or
1. Spawn as a civ.
2. Spawn a Panther and some kind of opfor vehicle so that the Panther can just lob grenades at it. Very randomly the explosions will either be normal or again very uncomfortably loud.
Really, whether its in MP or SP this issue is reproducible.