well, as for the dll files, the 707 uses a different system for sounds. The 727's sound system is mostly likely built into the gauges somewhere in the panel folder, and may not be located easily anywhere, instead integrated right in. Ror the sound folder differences, one of them is for things like switch flips, APU, window opening, and any cockpit sounds outside of the Gyros, engines, gear, and flaps. The 727 systems read the sounds in the FSX\Captain_Sim\727\sound folder, to play sounds that default aircraft do not have the capacity to have, where as the SimObjects\Airplanes\CS727_100 sound folder is the sound folder that is setup like the default style (except for the excellent sounds!) so that it may be played by the FSX sound processing engine as normal sounds, not requiring any external file. It also makes it easier for people to edit sounds, or replace it (if they didn't rock already), without having to copy or fix all the cockpit sounds on the new sound set.

I hope I've answered your question!