It might be a setting in Dragon Player? ....... sorry I don't use that app so not sure if such settings exist in it for the choice of audio device.
If Dragon is the only app throwing the fault then I suspect it is a Dragon setting for the default audio device .....
I just installed Dragon player and attempted to play an .mp4 video ..... it plays without sound here.
Checked in PulseAudio and apparently there is no sound stream ....... the file is good .... so something weird with Dragon it seems .... it plays an .avi file of the same video WITH sound!
Checked a bunch of video and one audio file ...... played all videos WITH sound except .mp4
Played .mp3 audio file too.
So ..... was the file you tried an .mp4?