Dual Mono Audio not working

[rsbrux]

This feature is quite important in Switzerland, where dual mono audio in different languages is quite common.
Unfortunately, I haven't been able to get it to work.
Dual-language programs, which my television or Panasonic MDR-EH80V can correctly recognize and select channels deliver only channel 1 (left) when played back in MP, regardless of which option I select from the "Audio Dual Mono"
2nd level context menu. Left & right channels produce the same audio; stereo is the same at lower volume.
What could be causing this? Are there any limitations or needed configuration of codecs which I should know about?
TV-Server Version: 1.0 RC2 SVN
MediaPortal Version: 1.0 RC2 SVN
MediaPortal Skin: BlueTwo wide
Windows Version: XP SP3 MCE RU2
CPU Type: Intel Pentium 4 @3 GHz
HDD: Samsung HD501LJ 500GB
Memory: 3 GB DDR 400 DIMM
Motherboard: FIC P4M-915GD1-LF
Video Card: Club HD3450 256MB DDR2
Video Card Driver: ATI Radeon 3450 8.493.0.0
Sound Card: Realtek High Def. Audio
Sound Card AC3: no AC3
Sound Card Driver: 5.10.0.5319
1. TV Card: Hauppauge HVR-1300 MCE
1. TV Card Type: hybrid (only analog section in use)
1. TV Card Driver: 2.119.25023.0
2. TV Card:
2. TV Card Type:
2. TV Card Driver:
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec: ATI
MPEG2 Audio Codec: Dscaler
h.264 Video Codec: ATI
Satelite/CableTV Provider: Telezug (WWZ) Switzerland
HTPC Case: FSC Scaleo E (FIC Spectra)
Cooling: OEM
Power Supply: OEM
Remote: PDAwin.com TVRC 5.5
TV: InFocus TD40
TV - HTPC Connection: DVI