To fix your speaker problem - Go to
Settings > Bluetooth > Turn On and wait for your list of devices to populate. Locate your speaker and Long Press it (press and hold) and then select
Un-pair. You'll be free to attempt to pair it again with the proper PIN!
If your device isn't paired with the phone, but it's still not connecting, you can try clearing your Bluetooth cache. Turn off Bluetooth, then Go to
Settings > Applications > All > Bluetooth Share and tap "Force Stop," then "Clear Cache," then "Clear Data. If the device was ever paired or a pairing ever failed, it may have stored those credentials regardless of success. This should give you a fresh chance at it.
As for why you can't specify what PIN your speaker provides your phone - it's because the default PIN is assigned to the device when it's made, and can't be changed by the average consumer. Most 'dumb' Bluetooth devices have no way of changing the PIN it comes with.