It is true that all Orthodox liturgies are sung, but the people don't necessarily sing everything not assigned to the clergy.
What is probably most common, at least in America is a mix of choir, solo chanters, and congregation.
There are a few parishes that try to have the congregation sing everything, but it frankly is terribly distracting, since we have so much variable material in our services, particularly at Vespers and Matins -- it requires a lot of photocopying and handouts, and doesn't seem to reflect the Orthodox liturgical ethos, IMHO.
The key to having the congregation sing along on the responses (and we have a lot of them), is that the fixed portions of the Orthodox Liturgy never change in text (that's just how our liturgies work) -- and wise chanters/choir directors stick to simple chant melodies and don't change them. That way, over time, those who regularly attend learn text and melodies alike by heart. I am personally of the opinion that having books for the congregation actually interferes with this process of memorization.
For the variable material, this is either sung by the choir, or by a solo chanter. This material is best prayed by the congregation via active and attentive listening. Over time, even some of the variable material is learned by heart (especially troparia). There are also a few portions of the services that are designed to be more melismatic, and often work best as set pieces for the choir or a solo chanter -- such as the Cherubic Hymn.
Ideal liturgies involve a balance of priest(s), deacon (if you're lucky enough to have one), tonsured reader, choir, solo chanter(s), and congregation. It is very natural and fluid.
There are some Russian churches where the choir does everything and the people are expected to be quiet -- same for some Greek churches with a master psaltis whom everyone is expected just to listen to. But it is a rare Orthodox parish, again, at least in America -- where a good portion of the congregation doesn't join in with ease on the fixed portions of the services...
Unless there is a choir, they would have to. At many of the Eastern Catholic Church Liturgies I have been too, especially among the Ruthenians, there is no choir, and the congregation was able to sing all the responses. In fact, there were two full years where I went daily on weekdays to Mass at an Eastern Church - Ruthenian in Pittsburgh and then later Ukranian in Wilmington, and being generally the only one in attendance on a weekday, made up the choir and congregation. Boy did I learn Slavonic chant quickly - both in Slavonic and English translation!
What is probably most common, at least in America is a mix of choir, solo chanters, and congregation.
As well it should be if possible. In the Roman Rite, the choir (St. Pius X calls them, "The Choir of Levites" to remind us it is really a clerical function) is to sing the proper Antiphons, and the lead the laity in singing the Ordinary.
The key to having the congregation sing along on the responses (and we have a lot of them), is that the fixed portions of the Orthodox Liturgy never change in text (that's just how our liturgies work) -- and wise chanters/choir directors stick to simple chant melodies and don't change them. That way, over time, those who regularly attend learn text and melodies alike by heart. I am personally of the opinion that having books for the congregation actually interferes with this process of memorization.
The Roman Mass is the same. The ordinary is fixed and doesn't change (other than the Creed and Gloria not being sung on weekdays except feasts), and the tones for responses are simple and unchanging. The tones for the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei are around 16 in number (although not all are used very often, some being proper only to Masses of the dead, or of the Blessed Virgin). There is a series of tones for Sundays, i.e. 1 for Easter, 1 for Sunday's after Pentecost, 1 for Advent, 1 for Christmastide, 1 for Lent, etc., and these one quickly learns from repition.
The books are only helpful when one is first learning to follow along. Otherwise, they should only be used for the following the readings and proper (changing) prayers.
For the variable material, this is either sung by the choir, or by a solo chanter. This material is best prayed by the congregation via active and attentive listening. Over time, even some of the variable material is learned by heart (especially troparia).
The learning of this material would be near impossible in the Roman Rite, where every Mass has its own individual chants. Only the most memorable parts, such as the Sequences (Victimae Paschali, Laud Sion, Stabat Mater, etc.), or a handful of commonly used antiphonal chants (such as Gaudeamus Omnes on many feasts of Our Lady), lend themselves to ready memorization.
Ideal liturgies involve a balance of priest(s), deacon (if you're lucky enough to have one), tonsured reader, choir, solo chanter(s), and congregation. It is very natural and fluid.
I don't disagree. For us, Priest, Deacon, Subdeacaon, Lector (for Masses with more than two readings), Cantor(s), Acolytes, Congregation.