Not sure if things have changed, Tascam updated the drivers to my us1641 in Dec 2014. 
 
It has always been the only audio device and that's with a few different computers and using W7, W8.1 and W10. I have all other audio devices disabled. I always use 44.1hz which I think is one reason it always works.
I can open Sonar, Wave Lab and WMP or Win Amp all at the same time no problem.  But--It won't play a video in VLC or WMP at the same time Sonar is open because those are often 48hz. 
So try setting Sonar to 48 and playing the video. 

Media sample rate does not matter on my PC. I can play videos that have audio at 48kHz while running SONAR at 44.1kHz. Tried with VLC 2.2.6 among other players. As mentioned above, I have used different interfaces (M-Audio, Presonus and RME) on different PCs running different OSes (XP, Win7 and Win10) all with the same results, But maybe it is an issue with Tascam devices.

scook
Media sample rate does not matter on my PC. I can play videos that have audio at 48kHz while running SONAR at 44.1kHz. Tried with VLC 2.2.6 among other players. As mentioned above, I have used different interfaces (M-Audio, Presonus and RME) on different PCs running different OSes (XP, Win7 and Win10) all with the same results, But maybe it is an issue with Tascam devices.

i suspect that those drivers that require the same sample rate in windows as the daw are somehow going via the windows audio stack, rather than real direct-to-hardware-asio which bypasses that? (c.f., asio4all)

The reason to have Windows and DAW sample rates the same is the interface can only operate at one sample rate at a time. If the sample rate is not set the same then the application currently using the interface prevents other applications running at a different sample rate from using the interface at the same time.

ok, i always thought asio worked a bit different as it's not the driver windows is using

It is different and most Windows programs do not use ASIO. Whatever driver an application uses still has to deal with the limitations of the hardware. AFAIK, no interface can work at multiple sample rates at the same time. Not having used a Tascam interface, I have no idea what other limitations they have.