avidemux clip missing both audio and video

running 2.6.10 on win7. have set both A and B on I frames; outputs set to copy. tried with audio input checked and unchecked. After save the resulting file is only 229924KB and doesn't play.

Is the output container (muxer) set correctly eg avi, mp4 etc ?

The muxer originally was avi and file saved with .avi extension. Changed the muxer to mp4 and it worked fine.