Nope, sorry; I don't have any recordings of Sharp-tailed, but the description in "Birds of Venezuela" is good. There is a recording of Bare-faced on the "Birds of Venezuela" cd (but it is obviously copywrited so I can't just pass it on). However, here are some comments, based on your second reply:
"flying quite high"
- I have never seen a "high flying" (though that is obviously a subjective statement; what is "quite high"?) Sharp-tailed Ibis. They are usually rather shy... and so is Green.
"pretty fast"
- Another subjective statement, but generally Sharp-tailed would be considered a slow flying ibis (contrary to the Bare-faced)... However this is compared to other ibis-sp., so, unless you have experiance with them, it is hard to say anything...
"heard it while it was flying about in a field"
- If you heard it at from any sort of distance, you can almost certainly rule out Bare-faced. Again, see my first thread on this subject.
"two of them"
- The Bare-faced Ibis is almost always found in big groups.
So, some of the above notes are obviously somewhat contrary! Are you sure you saw the red face (also note that Sharp-tailed usually has a small greyish patch on the chin)? - And, if so, are you sure the first individual you heard was the same species as the two you later saw flying high overhead? Did you see red on the head in both the first and also the two later individuals? Often several species of ibises occur together... I have seen 5 different species of Ibis together in one small pond (!) and, in the Llanos of Venezuala, Sharp-tailed and Bare-faced Ibis can sometimes be seen together. It is perhaps important to note, that Sharp-tailed rarely (if ever) occurs in big (mixed or uni-species) waterfowl aggregations.
Anyways, in short; if all your observations are right, I would guess that you saw two different species. The first lone calling individual being a Bare-faced (I am guessing that you saw the red on the head in this individual), and the second two high flying individuals would be Plegadis (it would be very hard to see any red throat patch if the birds are flying high. Also judging from you mentioning "white" in the flying birds). And, as mentioned previosly, a Plegadis seen in Guyana would almost certainly be a Glossy Ibis. Still, these are nothing but guesses, and you may very well have to make a second visit to the spot...
Last question; did you notice if the legs extented beyond the tail in the flying birds? If so, you can surely pin them down as Plegadis, as Sharp-tailed, Bare-faced and Green Ibis all have legs that DO NOT extent further out than the tail in flight.
Rasmus