This is a known problem of viDrop 0.55 - some Media Players won't play the sound on PC - but the files are mostly okay: you should hear the sound on your mp4-player.
Getting picture and sound staying in Sync is another problem: you could use the MEncoder Extra Parameters -Speed or -Delay - please do a little search about them here in the forum or go to
www.mplayerhq.hu to reads the MAN-Pages of MPlayer and MEncoder.