To be honest, I can't think of any reason why you would want to use tablet mode on a media PC with voice control but anyway I don't think that is really the problem here.
I don't think it's possible to focus a window while the Windows start menu is open. From my quick tests this morning this seems to be true whether or not you are in tablet mode.
Also, even when not in tablet mode and with the start menu closed, there are times when Windows decides that it won't let programs change focus. I'm not sure why exactly it does this but I'm sure that Microsoft will tell you that it's to "protect the user".
As for work arounds I think the most obvious solution is to close the start menu. You can get VoxCommando to emulate pressing the Windows key but the problem with that is that it will toggle the start menu which means it will open if it's closed. So probably you'd need a separate voice command to toggle the start menu.
Another (perhaps better option) would be to use the Escape key. This should close the start menu, and even if it has some unintented consequence depending on what you are doing it is probably not going to cause any serious problems.
Here is XML for both options:
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.2.4.3-->
<commandGroup open="True" name="focus tests" enabled="True" prefix="" priority="0" requiredProcess="" description="">
<command id="50" name="focus {1}" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>InputKeys.Send</cmdType>
<params>
<param>{ESCAPE}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Window.Focus</cmdType>
<params>
<param>{1}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>focus</phrase>
<payloadList>voxcommando, chrome, explorer</payloadList>
</command>
<command id="51" name="toggle start menu" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>InputKeys.Send</cmdType>
<params>
<param>{LWIN}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>toggle start menu</phrase>
</command>
</commandGroup>