In menu 'File' -> 'Open video file...' select 'image sequence' as file type. File names shoud have numbers at the end, like pic001.bmp, pic002.bmp, pic003.bmp,... Select first file and VirtualDub will load all of them like a regular video file. Then you can go to 'Video' -> 'Frame rate...' to set fps to whatever you want. In menu 'audio' you can add then audio track, from external WAV file (you can extract audio from avi to wav using 'File' -> 'Save WAV...' option).