Speaking loudly in any case is not liked and even this is considered as bad manner. One needs to be audible only and as long as you are being heard properly, you are at ease with audience.
On the pretext of showing leadership skills few MBA aspirants like to take lead and start speaking loudly in GD. Imagine the scenario that entire group is doing the same and that looks like shouting which is almost prohibited in GD. Up to the extent if it continues entire group may get rejected.
Ensure that in GD you are speaking at your turn with sound and audible voice with firm stance which you are putting forth based on assigned GD topic. During GD if you wish to interject, do so with permission of co group members.
Your rebuttals or elaborations must be backed up with sound logics so that you can get chance to lead the group.