Hallo Nassos,
ich weiß auch leider nur das, was das www hergibt.
Das Romeiko Ensemble hat eine eigene Webpräsenz:
http://www.romeikoensemble.com/
Darin heißt es u.a.:
The Romeiko Ensemble was founded in 1993 in Philadelphia (USA) and performs the Classical Music of Constantinople (Istanbul), namely the rich musical heritage of both the Byzantine and Ottoman empires.
...
Romeiko Ensemble, under the musical direction of Yorgos Bilalis and the vocal tutelage of master vocal technician John Nicholas Peters, has undertaken the task of presenting side-by-side authentic interpretations of the parallel Byzantine and Ottoman musical heritages (Christian and Sufi). Having carefully selected musicians skilled in historical performance practice and improvisation, Romeiko Ensemble has recorded unpublished medieval compositions based on thorough research of the musical manuscripts. The ensemble began its public performance career in 1994 and has traveled extensively throughout the US and Europe performing at festivals, gala concerts, church and tekke ceremonies and services as well as other cultural events. Romeiko Ensemble was catapulted to fame at the 13th International Festival of Orthodox Music in Hajnowka, Poland (1994), where the ensemble received "special distinction" among the 37 international choirs taking part. The ensemble was selected as the most authentic choral group from the US to perform at the Millennium Visit to Detroit of the Patriarch of Constantinople (2). Its discography includes: The Sounds of the Parthenon (1996), Thy Cross We Adore (2), The Sin of the Fig Tree (21), From Adam unto Joseph (22), Shall We Sing for You...? (23), Divine Liturgy in Mode I (26).
Ich wußte gar noch nicht, daß sie eine amerikanische Gruppe sind.
Gruß
c.
Gerade läuft eine Aufzeichnung der Vigilien aus der Kartause Parkminster, vorher lief Stundengebet aus der Abtei Mariendonk. 
Wie ich schon sagte, kommt da ja außerdem nicht nur Gregorianik, sondern auch mittelalterliche Musik und Musik der Renaissance.
