I am listening to a piece of music sung in maqam bayat (or bayati). It's Psalm 126, which is 7 verses long, but the piece takes about 10 minutes and sounds very improvisatory. This is similar to other examples of music in a maqam, so I'm wondering: is this type of music, where a soloist sings slowly, repeating words, without a repeated melody, mostly improvised, or is it mostly composed ahead of time and performed similarly at different performances, whether by the same singer or a different singer? I unfortunately know very little about how Arabic classical music works. It's possible that the example I'm citing, which is actually Jewish music from the Arabic world, is exceptional. Can anyone tell me more about Arabic classical music and improvisation?
A great deal of it is improvised, much in the same sense that Indian Raga is improvised. That is to say, a lot of melodic framework and development is predetermined, but there is a lot of room to work around the predefined bits.
http://en.wikipedia.org/wiki/Arabic_maqam explains this well.
I don't know about that peice in particular, but yes, there is a type of music where, as you said: a soloist sings slowly without much of a repeated melody.
In Arabic, it's called a "mawwal" (Arabic: موال). I know in Turkish it's "uzun hava" (according to what a Turkish friend told me). I'm sure it has different names in Krudish and Persian as well.
It's not always improvised. Sometimes it is, sometimes the melody is pre-determined.
Here are some examples:
There's a similar type of slow melodic type of recitation which is used (as far as I know) only for the Quran. It's called "tajweed" (Arabic: تجويد). And it's usually always improvised.