Venous return increases with inspiration, but the only useful ausculatory change is that right sided heart murmurs increase with inspiration (and of course splitting of S2). You need more physiologic changes to effect left sided murmurs (MR/MVP/HOCM/AS/AI) such as changes in LV size, systemic vascular resistance, etc.
Inspiration increases flow to lungs therefore can be used to differentiate right sided murmurs from left sided murmurs
Whatever increases blood volume in left ventricle worsens MR and decreases murmur of hocm and moves mvp click near S2
increasing TPR by squatting or something would increase the sound of mitral regurg, whereas with MVP you would want to decrease venous return to the heart by standing up, which would increase the sound of a MVP murmur, as per First Aid