Hi ClaireMac610
You haven't said which Panasonic TV you have, however if it has an HDMI port that is marked as Arc or eArc (probably HDMI 2) then connect the soundbar to that port, and the Stream box to the other HDMI port.
Enable 'One Power' in the Stream box settings (Settings > System > One Power) then the red power button on the Stream remote enables you turn on connected devices at the same time as the Stream box providing they are compatible and connected using an HDMI cable. The TV does need to have HDMI-CEC enabled for that function to work.
Since you haven't said which Panasonic TV you have you need to check for the following
(HDAVI Control, EZ Sync, or VIERA Link for CEC) and then go through the various menus to turn it on, which may look something like this
Press MENU on your Panasonic remote.
Use the directional buttons to highlight Setup.
Press OK.
Select HDMI control.
Select CEC function.
Select ON.
Have you set up your Samsung S700D soundbar correctly in the Stream settings?
The linked video below is for the 360 box but should work with the Stream box - forward to around 1 minute 7 seconds and follow the instructions to pair your Samsung soundbar with the Stream box/remote.
https://www.youtube.com/watch?v=rYmfJiLO7yQ
If the volume buttons aren't working with your on your 360 remote then double check under Settings > Audio Video > Volume Keys Control that it is set to Samsung S700D and not set to Panasonic (pressing the OK button on the Stream remote switches between them both)