These yells seem to me to be generally spontaneous exuberance rather than specific signals. Far from annoucing key changes, for eaxmple, they often greet the new tune, rather than announcing it.It is very noticable that loud hooching tends to occur when a Scottish band does the classic exciting key change from G to A.The stopped strings of G change to the open strings of A and the musicians and dancers explode with the pleasure of it.
In cajun however there is indeed some information often carried in the "aieeee" call.Fiddlers and accordionists often play different versions of the same tune when they take there solos, with different lengths of phrase, particularly in whether they include "hangs"(extra half-bars to extend the last note of the part of the tune). It is common practise to yell on this hang, which lets the accompanists know where the tune is going to start again. If you listen to old recordings you will hear the yells at these very speciific points: in an AABB type tune the yells tend to come at the end of the first A or B part, which is the point where the rest of the band will not necessarily know how the part is going to end: because it's the first time the soloist has played that bit.
On a related topic, I have had the pleasure of playing with a lot of Kurdish musicians recently. When the music is going well, the dnacers or drummers make an unbelievably thrilling ululation which makes the hairs on your neck stand up.Worth learning how to do it!