The first sound heard when taking the blood pressure is:

The first sound heard when taking the blood pressure is thesystolic pressure, not diastolic. When measuring blood pressure with a sphygmomanometer, the first Korotkoff sound indicates the systolic pressure. It is the pressure in the arteries when the heart beats (contracts) and is the higher of the two blood pressure readings. Diastolic pressure, which is the pressure in the arteries when the heart is at rest between beats, is indicated by the last Korotkoff sound heard.

References:

Manual of Hypertension of the European Society of Hypertension by Giuseppe Mancia

Blood Pressure Measurement Techniques - American Heart Assoc