Thanks very much for your very prompt reply and suggestion. Those video files are quite big and they are downloaded from some of the bt web sites on Chinese movies with both Mandarin and Cantonese sound tracks, e.g.,
Since it is not feasible for me to provide the video, I having provided the URL above.
As I said before, I merely chose 4.8 as I was not sure if 4.9 might have the same problem of not being able to switch from Audio 1 to Audio 2.
Thanks again for your help.