This all depends on how the bluetooth handshaking works in your PC.

In Win7: Control Panel | Hardware and Sound | Manage Audio Devices | Playback tab.

There SHOULD be a way to default to Bluetooth when the PC is paired to the speaker.

FWIW, I love the JBL Flip 3 bluetooth speaker we have. Couple it into a corner of the room and that little sucker puts out way more bass than one would expect.