audio syncing errors

i am trying to merge some audio and video so there are two channels of audio which are quite possible by various merging softwares such as mkvmerge which is used by me........ but sometimes the audio and video are not of the same length...........so someone suggested me to slow down the fps of the audio from 25 to 23.976 to increase audio length so that it synces perfectly .....so i was suggested the software megui but whenever i try to add any file it shows this:-


*img825.imageshack.us/img825/1910/20111223222619796x612.png

So....i also have sony sound forge 10 pro licensed version so if i could do something in it plz tell or suggest me something else