If I were teleported to the top of the Sphinx, could my voice get loud enough for someone to hear me?

The energy in a sound wave drops off with the square of the distance so the further away from the ultimate target, the less loud you will sound assuming you are already shouting as much as you can.

The only think getting above might do is give you a direct line of sight travel for the sound wave, instead of been absorbed by other people in the way. The you will have a improved chance of being heard because the energy is getting there direction as long as you distance to the target(s) is more or les the same - i.e. standing on a chair. But going the Sphinx would decrease the energy reaching the target by a lot and would make it less likely for you to be heard.

No...you'd be dead. Teleportation, as in "beam me up Scotty" is impossible due to quantum jitters. Scientific American had an interesting piece on this a couple of years ago. I suggest you look it up.

Now if you are more realistic and a helo drops you off atop that sphinx, the only thing that would change is the atmospheric pressure and density (it would lower) and then only a tiny bit. So sound would not travel as far as when back on the dessert floor, all other things equal. But, again, you'd probably not notice the diminution in range as it would be very slight.

So as the sphinx is not that high off the ground, I have no doubt your frantic calls could be heard below.

The Great Sphinx of Giza is only 20.2 meters high. Any healthy person should be able to shout loud enough to be hear at 20.2 meters, or even 100 meters.

It depends on where you were before teleportation, think along along the lines of explosive decompression. Taking this into conderation if you were to survive your vocal cords would probably be damaged.