No sound when playing the file from the media area (upper-left)? Or is there no sound only when playing from the timeline? If the latter, check that the MUTE button (at the left of the timeline) isn't active.
You say that mp4 plays okay?
MKV is a container for various file tormats. A codec may be missing. Try converting an MKV file to another format prior to loading into Videopad.
Try reinstalling the latest Videopad version.