Why does a partial or full avsd cause a holosystolic murmur? is it because associated valve regurgitation? thanks.

Turbulence: Murmurs are created by turbulent flow. A vsd creates turbulence as high pressure blood from the lv mixes with the rv.

Typically: Iin an avsd, there will typically be a systolic ejection murmur from increased flow across the pulmonary valve related to pulmonary overcirculation. The vsd shunting will often be low velocity and not produce an audible murmur, so any holosystolic murmur would usually derive from av valve regurgitation, a common problem in this defect.