Is the nebula the first source, or second source?
Are you sending to nebula, and nebula to BT?
If so, this would make a small problem bigger. Your BT to nebula is already out of sync, and then to send from nebula to BT sport, your magnifying the problem.
On some phones, you can change settings to sync better by changing the code/format of the signal.
Maybe if you used a cable to go from phone to nebula and BT the sound after, it MAY improve?