Maybe that is an improvement in the 2010 C series TVs as multiple audio channels can't be selected on my 2009 B series TV.
I've just read the manual for the C8000 and in the Media Play section (Page 35) it mentions that Multi Audio is not supported but it doesn't make it clear whether that is for USB files, files played over a network or both. I do know that the functionality between USB and Network Media Play on my TV is different so it may be the same on yours. I still believe the problem is with the TV and not Twonky as Twonky just streams the file as it is and it is down to the TV to be able to select different audio tracks.
Samsung LE37B650T2WXXU; Liteon 5055GDL+ HDD/DVD Recorder; Playstation 3; Iomega Screenplay HD 500GB; Buffalo Linkstation Pro LS-320GL (running Twonky 6.0.30, SqueezeBox Server 7.6 and acting as a Print Server)