我個5.1喇叭冇人声

我的DVD机係Hyundai TBH-PDVD142U(DVD+HIFI [6個]）
Functions : - Playback of DVD,SVCD,VCD,AudioCD,Mp3CD,JPgCD,Divx CD
- 5.1 Channels Surround Output
- Comprehensive Video Output with Progressive Scan Support

http://hk.image.auctions.yahoo.com/users/1/9/8/3/wingwing999hk-img600x450-1209964452___00329-5.jpg
但我播DVD时按“Dobly Digtal-EX/5.1"就有背景声但冇人声，點解？
（我已经在Menu中按咗5.1声道)