I hear DVD sounds but no dialogue

This seems to me to be an odd problem. I have just acquired a G4 iBook
which is running 10.4.11. I started watching a DVD but find I get all
the background sound well but none of the dialogue soundtrack. I went
through the Help section with a fine toothcomb but I can't find the
solution to my problem anywhere. On the controller drawer (the part
which you can hide if you so wish) there is a facility for choosing
'English 1, English 2, English 3' etc but that doesn't seem to make a
blind bit of difference. So I can hear all the music and sound effects
but no dialogue. What am I missing or where am I going wrong? Help
please,