The primary answer is that given already ie in windows control panel...sound uncheck the 'exclusive use' box. However, there is another setting, inside studio one 4 that i have, where studio one lets other applications share the audio system, or not. similar thing, check or uncheck a box.
Cannot recall exactly where, but I had a song up and got a middle sized dialogue box up with various settings, and this was one. However, in Windows 7 at least, you can have anomalous behaviour eg despite the above being done, and me finally getting youtube audio again, then going back to my song in studio one, the bass synth plugin wouldn't sound that was midi triggered from a midi keyboard. EDIT IT is in preferences...advanced then near the bottom of that list it asks if you want to release audio resources to share with other programs, so check this box...default is not releasing.
closing studio one down and starting it again brought the sound back out the speakers...then i wanted a youtube instructional video up, and got the audio error again!
It would seem, some programs in some environments don't play nicely. It is like the use of the ASIO driver is not being 'let go' of or something, in the way it needs to be. But, I have only had studio one 3 weeks, and the midi keyboard 2 days, so there's a bit of faultfinding before i give in on this front. I am not keeping the ipad handy just to see instructional videos as editing proceeds. This will yield! Probably.