Mantra to Get Blessings of Pitrus

Is initiation necessary to listen to this mantra?

No. Initiation is required only if you want to do mantra sadhana, not for listening. You just have to listen to the mantras we provide to get benefited.

Mantra 1
स्वादुषंसदः पितरो वयोधाः कृच्छ्रेश्रितः शक्तीवन्तो गभीराः ।
चित्रसेना इषुबला अमृध्राः सतोवीरा उरवो व्रातसाहाः ।।

Meaning:
The ancestors (Pitrus) who delight in sweet offerings are powerful and strong. They have faced hardships, yet are full of strength and wisdom.
Their army is colorful, their weapons are arrows, and they are undefeatable. They are victorious, broad-minded, and conquerors of enemies.

Mantra 2
ब्राह्मणासः पितरः सोम्यासः शिवे नो द्यावापृथिवी अनेहसा ।
पूषा नः पातु दुरितादृतावृधो रक्षा माकिर्नो अघशंस ईशत ।।

Meaning:
The ancestors who were scholars, dedicated to Soma (divine nectar), pray for the well-being of Earth and Heaven, both free from harm.
May Pusha (a Vedic deity) protect us from distress and evil. Let the righteous protect us, and may no harm or evil thoughts control us.

Mantra 3
सुपर्णं वस्ते मृगो अस्या दन्तो गोभिः सन्नद्धा पतति प्रसूता ।
यत्रा नरः सं च वि च द्रवन्ति तत्रास्मभ्यमिषवः शर्म यंसन् ।।

Meaning:
The sharp-toothed beast (a tiger or similar animal) bears bright feathers, and its teeth are like weapons. Wrapped in its hide, it moves swiftly.
Where men rush together in fear or flee in panic, let the arrows of protection shield us in that place.

Ramaswamy Sastry and Vighnesh Ghanapaathi