hi , i think it would be very difficult to convert Pronto hex to a wav file reliably.

the Pronto Hex file contains a modulation frequency, and a Pair of burst code pairs, and end pulse and some times a repeat code, there is a great article on Pronto HEX codes by Barry Gordon,it should be your starting point.

If your doing this for fun, then have fun... but there are many produts already in the market place.

Just a comment , you can not drive IR LED's via the audio jack you will need some form of driver like and FET..