tried posting on this thread earlier but the forum wasn't working
inhale through nose, exhale through the mouth in boxing is how i've always learned it. I have nose problems so end up mouth breathing a lot, which is bad.
Sounds, if you make any, are either a hissing sound or a grunt.
Some guys like Pacquiao grunt on every shot.
Some folks just hiss.
Some folks make no sounds at all.
i usually avoid weighting a shot at the end of a combo- I give everything aside from jabs good power, which comes from my feet & balance shift; keep it fast, sharp, and connected.
holding your breath on a power shot can definitely cause problems under the right/wrong circumstances. as long as you exhale you should be fine.