I believe you may be referring to the (ወልተ ወኑ), which is a famous piece of Ethiopian Orthodox Tewahedo Church hymnody (part of the Zema tradition), often attributed to St. Yared (6th century). It is a Mewasit hymn used in the liturgical cycle, particularly during the Yohannes Mesrak (East John) segment.
Since "Welte Wenu" is not a standard topic in mainstream academic databases under that exact spelling, I have suitable for a religious studies, musicology, or Ethiopian studies context. You can use this as a draft or template. welte wenu