Question about Hauppauge TV tuner WinTV-dualHD USB stick DVB-C/T2/T with FB

The manufacturer does not specify this, sorry.

Even if the question is already old: In Plex I can select all sound tracks, even a 5.1 track (if available). I have not tested whether the Windows app can do the same.

Yes. With Tvheadend, video and audio is recorded/displayed exactly as it is received. ORF and SRF often use AC-3 5.1 Surround, but it may not be the default track (e.g. track 1 may be AAC German, track 2 AC-3 5.1 Surround German, track 3 AC-3 2.0 English). So the hardware is capable, it depends on the software.