Where can I find old cyanogenmod notification sounds?
As by the very same answer @Firelord is referring to in his comment on your question, rather look under /system/media/audio/notifications :) Well, basically, for all those sounds (ringtones, notifications, alarms, …), simply look below /system/media/audio/*.

As the path suggests, those should be in the system.img (name may slightly differ, but surely has "system" in it).

To prevent their "disappearance" on future flashes, I'd recommend placing those sounds you wish to keep on your SD card, e.g. below /sdcard/media/* (corresponding paths). Android then finds them as well – and a wipe/flash for future Android version installs wouldn't remove them from there (unless you chose to wipe the card – which isn't required for updates).