DESCRIPTION:
This psalmody from the vespers is chanted wonderfully by the monks of Chevignon monastery. It is sung in the Old Slavonic liturgical language of the Slavic Byzantine Catholic Churches. Many thanks to user Giedrius Misiukas who identified it as Psalm 141:3-4. Below is the text (in cyrillic) and an English translation:

Положи, Господи, хранение устом моим и дверь ограждения о устнах моих.
Не уклони сердце мое в словеса лукавствия, непщевати вины о гресех с человеки делающими беззаконие: и не сочтуся со избранными их.
Гласом моим ко Господу воззвах, гласом моим ко Господу помолихся. Пролiю пред ним моленiе мое, печаль мою пред ним возвещу. Внегда изчезати от менe духу моему, и ты познал eси стeзи моя
Воззвах к тебе, Господи, рех; ты eси упованiе мое, часть моя eси на земли живых. Вонми моленiю моему, яко смирихся зело; избави мя от гонящих мя, яко укрепишася паче менe.

Set a watch, O Lord, before my mouth; keep the door of my lips. Incline not my heart to any evil thing, to practise wicked works with men that work iniquity: and let me not eat of their dainties. I cried out to God for help; I cried out to God to hear me.
I cry aloud to the Lord; I lift up my voice to the Lord for mercy.I pour out before him my complaint; before him I tell my trouble. When my spirit grows faint within me, it is you who watch over my way. I cry to you, Lord; I say, “You are my refuge, my portion in the land of the living. Listen to my cry, for I am in desperate need; rescue me from those who pursue me, for they are too strong for me.