Add SoundGroup:GetSounds()

SoundGroup are amazing they are really useful for controlling a lot of related sounds at once but I think it currently lacks a function that returns all Sounds that belongs to a SoundGroup.

Because we would put SoundGroups under SoundService which is recommended by the wiki, and the Sounds under said SoundGroup, in cases where we want to Play a Sound in a 3D space we can no longer do this, which is why SoundGroup:GetChildren will not suffice for this use case.

However a simple alternative to this would be using CollectionService but it would definitely be nicer to have this functionality built into SoundGroup for simplicity.

Additionally SoundAdded and SoundRemoved would also be beneficial to have