I will only speak from my experience with breathing and sound in Hung Gar as the breathing is different in Pak Mei. As mentioned above, the sounds in Hung help one develop and feel different patterns of contraction and expansion within the body, as well as at different speeds and amplitudes. Once one has trained these methods for a while, a few notable changes begin to become apparent:
1) One becomes aware of how breathing and power intertwine to a much deeper degree. A movement can create a breath/sound, and a sound can create a type/quality of movement. In truth they are the same thing.
2) One begins to develop the "lions roar", and the voice and breath can be used in a very sharp, sudden way. When combined with movement (say taking a step or flinching, this acts a bit like a super charger acts on an engine. There are studies I could dig up that show how projected audio vibrations can cause one on the receiving end to react more slowly/sub-optimally and I suspect the the concept of a Kiai in Japanese arts, as well as the use of sounds in martial arts in general often ties in some with this. This study also shows some of how sound can help with athletics: "Researchers from the University of Nebraska have recently published a study in the Journal of Strength and Conditioning Research which may offer some scientific foundation for the physical benefits of kiai (or as they word it ”grunting”.
They discovered that grunting produced a 3.8% increase in groundstroke velocity for tennis players, while not impairing any oxygen consumption." Combine that with the studies showing how one can exert more power more efficiently when the tongue and neck are engaged in certain ways and you have some interesting concepts to work with
(Having developed a bit of the lions roar makes arguments escalate quickly if one is not mindful as the voice begins to project and raise quickly and often more forcefully than it feels like one means to use it.)
3) The vibrations made by different tones, as well as the way the body moves to create the sounds themselves can resonate or guide the intent and awareness to parts of the body to help relax, recruit, or change how/if we move them.