“Gee,” said Gene. “When I was a kid in Hebrew school, we used to sing ‘Sim Shalom’ with a great tune, echoing the ‘sim’ part! What happened to that tune?” Then Gil replied, “You never even went to Hebrew school! But you’re right. I remember a totally different ‘Ledor vador’ at the end of the Kedushah, and I miss it. What happened to the great tunes?”
Well, Gene and Gil, there are a couple answers to your questions. The short answer is that most likely the tunes didn’t go anywhere.
For sung prayers (“congregational melodies” - those which the congregation joins in singing) such as “Sim Shalom,” “El Adon,” and “Adon Olam,” for instance, there are many tunes that may be sung. The prayer leader often has choices among the tunes and may lead with one tune today and a different tune tomorrow. (Your correspondent’s favorite Psalm is 150, and there are several great tunes for that one.) Rabbi Adelson chooses from his vast and ever-increasing storehouse and may surprise you.
Almost all of the congregational melodies were written since the 1850s, and the vast majority are after World War II.
Maybe your favorite tune is not in the current repertoire of the leader or isn’t a favorite, and maybe a bit of teaching or nudging might produce it once in a while. Maybe that part of the service is left out at the times you attend, but if you only would attend at other times - say Friday evening or Wednesday morning - you would hear it.
A different answer to your question, however, might be that you are attending when the nusah - the prayer chant melody - is different from what you remember. The cantors over the years developed different chant melodies for different services at different times of different days, such that had you been Rip Ben Weinkle awakening from a long slumber and you stumbled into a service, you would know exactly what day of the week and time of day it was, or maybe whether it was a holiday or not.
So possibly your reduced attendance - especially if it is reduced only to Rosh Hashanah and Yom Kippur each year - exposes you only to the nusah for those days, and you do not get to hear the melodies you remember. Generally, a prayer leader would not break into a rousing “Ledor vador! (Ledor vador!) Ledor vador! (Ledor vador!)” in the middle of Yom Kippur morning (unless there was a special reason). But it might occur at musaf on Shabbat. (We remember fondly here Sylvan Simon z”l who loved that version and wanted to hear it at the end of every Kedushah.)
Here in Pittsburgh we have a wealth of prayer leaders with a vast knowledge of tunage. Keep coming to services, and it is likely you will hear your favorites again! Or at least then you will have a better standing from which to request them.